The 2026 Guide to Multilingual SEO for International Growth

The 2026 Guide to Optimising a Multilingual Website: Succeeding With International SEO

In 2026, optimising a multilingual website for search is no longer a "nice to have" reserved for large enterprises. It is a direct growth lever: capturing local demand, protecting your positions against international competitors, and remaining visible in SERPs increasingly shaped by AI. The challenge is not merely linguistic; it is also technical (URLs, hreflang, indexing), editorial (intent by market), and operational (governance, quality, scale).

This guide focuses on international implementation and ongoing management (without delving into the general fundamentals of SEO). The aim: to give you a practical, measurable, sustainable framework, whether you operate in B2B, e-commerce, SaaS, or media publishing.

Definition, Scope, and Use Cases (B2B, E-commerce, SaaS, Media)

Multilingual SEO aims to improve a site's visibility in target markets by optimising content across multiple languages (and often multiple countries). The objective is to increase qualified organic traffic internationally and, by extension, business impact; according to Intertranslations, this approach helps to "increase your website's visibility abroad" and boost the flow of organic traffic.

The scope goes far beyond translation: you must adapt queries, proof points, tone of voice, and expected formats, and set up the technical "signposting" (URL structure, hreflang, sitemaps, canonicals) so search engines understand which version to show to which user.

• B2B: capture industry queries by country (regulation, sector terminology, sales cycles) and generate qualified leads via solution pages and expert content.
• E-commerce: localise categories, filters, product pages, help content, and reviews; each market has its own language and trust signals.
• SaaS: adapt your value proposition (use cases, integrations, pricing), comparison pages, and documentation to local expectations.
• Media: optimise discoverability and topical depth by local edition, whilst avoiding cross-language cannibalisation.

Why a Multilingual Approach Becomes Strategic in 2026: International Growth, AI, and SERP Competition

Two trends make localisation non-negotiable:

• Users' language preference: according to SEO.fr, more than 7 in 10 internet users only spend time on sites in their native language, and only 42% will buy from a shop that is not available in their language.
• Competition and click concentration: according to SEO.com (2026), position 1 captures roughly 34% CTR on desktop, and the top 3 about 75%. Conversely, Ahrefs (2025) indicates page 2 receives only around 0.78% of clicks. So an "average" international rollout (translation without optimisation) becomes invisible quickly.

Now add AI and "zero-click": our SEO statistics and our GEO statistics remind us that a significant share of searches end without a click (Semrush, 2025), and that structured formats (lists, FAQs, clear hierarchy) increase the likelihood of being reused by generative engines (State of AI Search, 2025). To stay visible, you need local relevance and "quotability".

Targeting by Language, by Country, or a Hybrid: How to Choose Correctly

Three approaches coexist:

• By language (e.g. French, English, Spanish): useful when intent changes little by country, or when distribution is global (e.g. SaaS).
• By country (e.g. France, Canada, Switzerland): essential when vocabulary, prices, availability, legislation, or competition differs significantly.
• Hybrid (language + country): often the best compromise for major markets (e.g. fr-fr, fr-ca, en-gb, en-us), provided you can manage the architecture and hreflang properly.

Version Internationale emphasises that a strong strategy relies on adapting "country by country", taking local volumes, habits, and user behaviour into account. That is what separates a "translated" site from an effective international strategy.

Setting the Strategy: Languages, Countries, Intent, and Business Priorities

Choosing Between Language Targeting and Geographic Targeting (and When to Combine Both)

Decide using data, not intuition. In practice:

• Analyse your existing traffic by country/language (Google Analytics/GA4) and your impressions/clicks by country (Google Search Console).
• Identify markets where conversion is already strong but traffic is low: these are often quick wins.
• Check whether one language covers multiple markets with different expectations (e.g. French in France vs French in Canada). Version Internationale highlights these lexical and cultural variations, which directly change queries and SERPs.

When differences are meaningful (pricing, delivery, legislation, social proof), combine language + country to avoid overly generic pages that convert poorly.

Keyword Research by Market: Volume, Intent, Competition, and Conversion Potential

The key point internationally: do not translate keywords, localise them. Intertranslations and Acolad stress that language-based SEO is specific: a literal translation is easily "lost", because people do not search using the same terms.

Recommended process (by country):

1. Build a query set via Google Keyword Planner, Ahrefs, Semrush, and Google Trends (Version Internationale).
2. Classify intent (informational, navigational, transactional, commercial) and map it to the right page type.
3. Estimate business potential: basket value, sales cycle, margin, LTV, volume × expected conversion rate.
4. Analyse local competitors: dominant content, formats, backlinks, and gaps/niches (Version Internationale).

Practical tip: start with a batch of "core business" pages (home, product/offer pages, top-performing content) and validate impact before scaling (WPML).

Mapping Pages and Journeys on Your Site: Which URLs to Create, Merge, or Remove

Internationalising without a map quickly creates noise: duplicate pages, cannibalisation, inconsistent slugs, broken internal links across languages. Work in a "page → intent → market" model:

• Create: new pages when local intent differs (terms, proof points, use cases, availability).
• Merge: when multiple pages target the same intent in a given language.
• Remove / noindex: pages with no local demand, low-value filter pages, technical duplicates.

This "one page per intent" principle is a powerful safeguard, especially when content volume explodes across markets.

Governance and Planning: Making International Production Part of a Scalable Editorial Strategy

Multilingual programmes rarely fail because of a single technical issue; they often fail due to weak governance. A scalable set-up relies on:

• Prioritisation by impact (traffic, conversion, business value) and effort (translation, QA, development).
• Clear roles: SEO lead, native linguists, subject-matter validation, proofreading, publishing.
• Templates and guidelines by page type (category, product, guide, FAQ, landing page).
• Continuous quality control: terminology consistency, intent alignment, localised proof points.

Intertranslations also describes dedicated teams combining specialist "web" translators and SEO experts, with a final optimisation validation step. It is a sound model: separate production from QA, without unnecessary back-and-forth.

Implementing Multilingual SEO: Architecture, Technical Set-up, and Rollout

URL Architecture: Subfolders, Subdomains, or Country Domains (Decision Criteria)

Three architectures dominate (SEO.fr):

• Country domains (ccTLDs): example.fr, example.de. Strong geo-signal, but expensive to maintain and build authority for (Acolad).
• Subdomains: fr.example.com. Flexible, but authority may be split less naturally.
• Subfolders: example.com/fr/. Often recommended to consolidate authority on a single domain (Acolad), and simpler to operate.

Avoid language parameters in URLs (e.g. ?lang=fr) and displaying multiple languages on one page: SEO.fr explicitly advises against both, as they harm search engine understanding and user experience.

URL Rules: Consistency, Readability, Parameters, and Filter Management

Rules that hold up at scale:

• Localised slugs (not just translated page copy): WPML highlights the importance of adapted, consistent slugs.
• Short, explicit URLs and a stable structure over time (Version Internationale).
• E-commerce filters: control indexing (noindex, canonicals, crawl rules) to avoid thousands of low-value URLs consuming crawl budget.

Language Selector: UX Best Practices That Prevent SEO Mistakes

Your language selector should help, not trap:

• Use a visible selector (header or footer), with clear labels (language + country where needed).
• Avoid automatic redirects based on browser language: SEO.fr describes this as a bad idea (expats, browsers set to a different language, blocked access to the correct version).
• If you suggest a version (banner), always leave the choice to the user and keep a reachable URL for every version.

Hosting, Performance, and CDN: What Actually Matters for International Audiences

For international audiences, performance becomes both an SEO and conversion issue. Google (2025) reports that 53% of users abandon on mobile after 3 seconds. HubSpot (2026) links +2 seconds of load time to +103% bounce rate.

Pragmatic priorities:

• CDN to bring assets (images, JS, CSS) closer to users.
• Mobile optimisation (Core Web Vitals): SiteW (2026) indicates only 40% of sites pass the assessment.
• Template stability: in multilingual set-ups, a broken script or template is replicated across every language.

Hreflang, Canonical, and Indexing: Serving the Right Version and Avoiding Duplicates

Hreflang tags, canonicals, and sitemaps form a trio: they prevent users being sent to the wrong version and reduce duplicate content risks. WPML stresses consistency between hreflang, canonicals, and URL structure, otherwise signals are diluted.

The Role of Hreflang Tags (and Their Limits)

hreflang tells search engines which language (and optionally country) a page targets. It does not "magically" improve rankings, but it helps show the correct version to the right user and prevents language variants competing unnecessarily.

Important limitation: hreflang is not a fix for weak content. If the localised page adds little value (literal translation, missing proof points, misaligned intent), it will still struggle.

Implementation: HTML, XML Sitemap, or HTTP Header

There are three methods (Google Search Central):

• In HTML (link rel='alternate' hreflang='...'): common in CMS environments.
• In the XML sitemap: useful for scaling and auditing. WPML notes that if each language sits on a separate domain/subdomain, each host needs its own sitemap.
• In the HTTP header: relevant for non-HTML files (PDFs), less common.

Critical Cases: x-default, Regional Variants, Orphan Pages, and Similar Content

• x-default: useful for a "selector" page or a neutral version when no targeting applies.
• Regional variants: fr-fr vs fr-ca requires real lexical adaptation and sometimes distinct pages (Version Internationale).
• Orphan pages: if a local version is not linked anywhere (missing internal links), it may remain poorly discovered.
• Very similar content: if two pages (e.g. en-gb and en-us) are nearly identical, clarify differentiation (pricing, units, delivery, proof points, examples) and monitor cannibalisation.

Redirects, Noindex, and Canonical: Consistency Rules to Preserve Signals

Simple rules that are often broken:

• A canonical URL should point to an indexable page that matches the targeted language/country.
• Avoid aggressive geo-based redirects: they break access to versions and complicate crawling (SEO.fr).
• If you set a page to noindex, do not declare it as the canonical for an indexable page.

Content: Translation Is Not Enough — You Need Localisation

Translation vs Localisation: Vocabulary, Proof Points, Units, Legal Constraints, and Context

Intertranslations and Acolad are clear: an international strategy is not limited to translation. Localisation means changing what influences trust and conversion:

• Vocabulary (terms people actually use, synonyms, regional phrasing).
• Proof points (certifications, standards, labels, local use cases).
• Units (currency, taxes, date formats, measurements).
• Legal constraints (notices, cookies, warranties, country compliance).
• Cultural context (tone, formality, arguments, objections).

For e-commerce, SEO.fr also reminds us that raw machine translation is risky (nuance, context) and can damage brand perception, engagement, and performance.

Adapting Titles, Meta Descriptions, Headings, and Media Without Losing Search Intent

Do not limit localisation to the main body copy. WPML lists commonly forgotten elements: SEO titles, meta descriptions, slugs, alt attributes, and microcopy (menus, footers, sidebars). A partially translated site confuses both search engines and users.

On-page best practices by market (Version Internationale):

• Title tags and Hn headings built on localised, natural, intent-driven phrasing.
• Meta descriptions written for clicks (CTR), not translated word for word. Our SEO statistics indicate that an optimised meta description can increase CTR (MyLittleBigWeb, 2026).
• Images and media: localised alt text, locally relevant examples and visuals (formats, contexts, regulation).

Structuring Content for Search and AI Engines: Answer Blocks, Facts, and Quotability

In 2026, content must serve two readers: humans and summarisation systems. With rising zero-click (Semrush, 2025) and AI-powered overviews, scannable formats matter more.

To increase your chances of being reused:

• Start with a clear answer (operational definition, steps, decision criteria).
• Use lists and a clean hierarchy (H2/H3): State of AI Search (2025) associates this structure with higher citation likelihood.
• Support with data (studies, figures, named sources) and adapt them by market where needed.
• Create FAQs per language: long-tail queries matter, and SEO.com (2026) indicates 70% of searches are longer than 3 words.

Editorial Operations: Workflow, Proofreading, Glossaries, and Subject-Matter Validation

International quality is a process. WPML recommends a glossary to ensure brand consistency (product names, terminology) and, where relevant, to include target terms by market.

Recommended workflow:

1. Define the reference page (source language) and its objectives (intent, proof points, conversion).
2. Localise keyword research and adjust the angle by market.
3. Translate/localise with native linguists, then run an SEO QA (titles, headings, slugs, internal links, alt text).
4. Subject-matter validation (promise, compliance, offers) before publishing.
5. Measure and iterate by country.

Authority by Country: Link Building, Trust, and Brand Signals

Building Locally Relevant Links: Media, Partners, and Industry Directories

An international site rarely has the same credibility everywhere. Backlinks remain a strong signal: Backlinko (2026) indicates 94–95% of pages have no backlinks, and that the #1 result has on average around 220 backlinks (order of magnitude). Without local (or at least regional) link building, you may hit a ceiling.

Pragmatic approach:

• Local PR and industry media.
• Partners, integrations, resource pages (co-marketing).
• High-value local pages (guides, studies, comparisons) that are more linkable than purely commercial pages.

Internal Linking Across Languages: Hubs, Cross-links, and Prioritising Business Pages

SEO.fr recommends linking pages that offer the same content in different languages: it helps indexing and signals the existence of relevant versions. WPML adds a critical point: when translating, internal links should point to the translated version, not the source language; otherwise you lose relevance and harm the experience.

Architecture tip: build hubs (pillar pages) per market and link them to business pages (categories, offers, pricing), then to supporting content (FAQs, guides).

E-E-A-T Internationally: Brand, Authors, Trust Pages, and Entity Consistency

E-E-A-T must be localised too:

• Trust pages translated and adapted (About, Contact, legal notices, privacy policy).
• Identifiable authors and reviewers where relevant (local expertise, sector credibility).
• Entity consistency: product names, brand, addresses, numbers, and stable terminology via a glossary.

Multilingual SEO on WordPress: Plugins, Compatibility, and Key Watch-outs

Deployment Models: One Site, Multisite, or Multiple Sites

Three models are common:

• A single multilingual WordPress site: simpler to maintain, good for subfolders/subdomains.
• WordPress multisite: useful if each country has different teams, catalogues, or templates.
• Multiple sites: relevant for highly autonomous brands/countries, but more costly (SEO, tracking, governance).

Whatever the model, keep one rule: the technical approach should serve the strategy (market priorities, intent, conversion), not the other way round.

Configuring the Yoast SEO Plugin: Metadata, Mark-up, Indexing, and Template Quality

Yoast SEO structures your templates (titles, meta tags, indexing). In multilingual set-ups, the biggest risk is duplicating default metadata across all languages. Check:

• Each language has localised titles/meta descriptions, not identical variables auto-translated.
• Archives (categories, tags) are controlled to avoid mass-indexing weak pages.
• Structured data (schema) remains consistent across languages (Acolad).

WPML + Yoast: Settings to Validate Before Going Live

With WPML + Yoast, systematically verify:

• Localised, consistent slugs.
• Internal links point to the correct language (WPML).
• Canonical and hreflang tags are aligned (no mixing across hosts/structures).
• Translation of "invisible" zones (titles, meta tags, alt text), which are often missed.

Polylang + Yoast: Common Pitfalls (Taxonomies, Categories, Slugs, and Templates)

With Polylang + Yoast, common pitfalls include:

• Shared vs translated taxonomies (risk of inconsistent archives).
• Non-localised category slugs (lost relevance and poor UX).
• Templates injecting identical meta tags across all languages.

TranslatePress + Yoast: Managing Metadata, Rendering, and Performance

TranslatePress can simplify "visual" translation, but watch out for:

• Whether it can properly manage titles/meta descriptions per language.
• Rendering and performance (additional scripts, mobile impact).
• URL consistency (avoid mixed or ambiguous structures).

Weglot: What to Check (URLs, Indexing, Selector, and Translation Quality)

Weglot is often chosen for speed of deployment. Key checks:

• URL structure (ideally clear subfolders or subdomains).
• Language indexing (avoid accidentally blocking versions).
• Language selector (avoid uncontrolled automatic redirects).
• Translation quality: correct business pages and localise queries (Acolad, Intertranslations).

Yoast Sitemaps in a Multilingual Environment: Requirements, Checks, and Common Errors

Your sitemap is a management tool: it helps search engines discover your URLs. WPML highlights a key point: if languages share the same host, one sitemap can cover all languages; if each language is on a separate subdomain or domain, each host must provide its own sitemap.

Checklist:

• The sitemap includes only indexable URLs (no 3xx, 4xx, 5xx, and no noindex pages).
• Listed URLs match what is actually published (no partial translations).
• Hreflang is consistent with the declared URLs.

Mistakes to Avoid: A Go-live Checklist for an International Site

Hreflang Errors: Reciprocity, Codes, Final URLs, and Inconsistencies

Hreflang errors are costly because they break country targeting. The most common:

• Missing reciprocity (A points to B, but B does not point back to A).
• Invalid or inconsistent language/country codes.
• Hreflang pointing to redirected or non-indexable URLs.
• Misalignment between hreflang and canonical tags (WPML).

Mixing Multiple Languages on One Page or Publishing Partial Translations

SEO.fr advises against displaying multiple languages on one page. WPML adds that mixing languages (including menus, sidebars, footers, and microcopy) "confuses" both visitors and search engines and hurts performance. Publishing partial versions can also damage trust and conversion.

Literal Translations: Lost Intent, Cannibalisation, and Lower Conversion

Version Internationale notes that the same term can have multiple equivalents depending on context. A literal translation can:

• Make you miss the terms people actually search for.
• Create cannibalising pages (two pages target the same intent poorly).
• Reduce conversions because the messaging does not resonate locally (WPML).

Overly Aggressive Geo-redirects and Crawling Blocks

Automatic IP/browser-based redirects are risky: they block access to versions, make testing difficult, and can hinder crawling (SEO.fr). Prefer a non-blocking suggestion and a clear selector.

Launch Checklist: Crawl, Indexing, Performance, Tracking, and Content QA

• Crawl: check robots.txt, sitemaps, HTTP status codes, redirect chains.
• Indexing: key pages indexable, canonicals consistent, hreflang validated.
• Performance: mobile first, Core Web Vitals, multilingual template weight.
• Tracking: GA4 + GSC correctly segmented by country/language.
• Content QA: translated titles/meta tags/slugs/alt text, internal links to the right versions, localised proof points and units.

Measuring Performance: KPIs, Attribution, and ROI Management

Defining KPIs by Country and Language: Visibility, Qualified Traffic, Leads, and Revenue

Measure by market, otherwise you are managing blind. Recommended KPIs:

• Visibility: impressions, positions, share of top 3 rankings by country/language.
• Qualified traffic: organic sessions, pageviews, engagement, bounce rate.
• Business: leads, conversion rate, organic CAC, SEO revenue (when trackable).

To connect effort to value, use a structured ROI approach (see SEO ROI).

Setting Up Tracking: Search Console, Analytics, and Market Segments

Minimum set-up:

• Google Search Console: properties aligned to your architecture (domain or URL prefix), tracking by country and queries.
• GA4: segments by country, language, directory/subdomain, and harmonised conversion events.
• Dashboard: market-by-market reporting to compare speed of progress, costs, and results.

Analysing Results: Cross-language Cannibalisation, Winning Pages, and Opportunities

Three analyses that lead to actionable decisions:

• Cross-language cannibalisation: one version ranks in the wrong country, or two versions compete in the same SERP.
• Winning pages: pages that reach the top 3 (remember: SEO.com 2026 estimates ~75% of clicks go to the top 3). Replicate the patterns (structure, proof points, format) in other markets.
• Opportunities: queries close to the top 10, pages gaining momentum, low-competition local niches.

Prioritising Optimisations: Impact × Effort × Risk, Market by Market

Use a simple prioritisation model:

• Impact: potential uplift (traffic, conversion, value).
• Effort: development + production + QA.
• Risk: regression, loss of indexing, template instability.

Internationally, low-effort/high-impact fixes are often: inconsistent hreflang/canonicals, duplicated metadata templates, misrouted internal links, and partially translated pages.

2026 Trends: How International SEO Is Evolving

More Volatile SERPs: Quality Requirements, Differentiation, and Continuous Updates

According to SEO.com (2026), Google rolls out 500 to 600 updates per year, and HubSpot (2026) mentions 200+ ranking factors. Internationally, this means:

• More frequent iterations on key pages (refreshes, proof points, UX).
• Market-specific monitoring (a German SERP does not evolve like a French SERP).
• More structured, more evaluable, more up-to-date content.

AI-assisted Search: Sources, Consistency, Reliability, and Trust Signals

Generative engines heavily cite the best organic results: our GEO statistics show that 99% of AI Overviews cite the organic top 10 (Squid Impact, 2025), and that ChatGPT citations strongly overlap with Bing (Squid Impact, 2025). The takeaway: "classic" SEO remains a prerequisite, but clarity, sourcing, and consistency become even more decisive.

Scaling With a "Quality First" Mindset: Automation, QA, and Governance

Scaling is the norm: the more markets you add, the more complexity increases (WPML). The 2026 challenge is automating what is repeatable (deployment, QA, templates, tracking), whilst raising the quality bar for business pages and high-visibility content.

Choosing the Right Support: International SEO Agency or International Consultant?

When to Choose an International SEO Agency to Accelerate Rollout

An international SEO agency makes sense if you need to:

• Launch multiple countries quickly, coordinating language, SEO, and technical work.
• Build local link acquisition (media, partners, PR).
• Scale production and QA (workflows, guidelines, calendars).

When a Multilingual SEO Agency Is a Better Fit (Production, Localisation, Multi-country Coordination)

A multilingual SEO agency is a logical choice when your main bottleneck is production and adaptation: localising intent, copywriting/transcreation, on-page optimisation, template alignment, and coordination with internal teams (product, legal, development).

When an International Consultant Is Preferable (Audit, Framing, Coaching, Governance)

An international SEO consultant is often better if you already have resources (copywriting, dev, translators) but lack:

• Architecture + hreflang + indexing framing.
• A market-by-market prioritisation method.
• A management system (KPIs, QA, governance, decision-making).

Evaluation Framework: Methodology, Deliverables, Management, and Transparency

Assess a partner based on tangible deliverables (Intertranslations):

• A full audit (technical, semantic, competitive) by market.
• Localised keyword research + intent/page mapping.
• An implementation calendar + prioritisation.
• Linguistic and SEO QA processes (checklists, acceptance criteria).
• Market-level measurement and the ability to connect outcomes to business value.

Speed Up Auditing and Rollout With Incremys

Diagnosing Technical, Semantic, and Competitive Gaps With the Incremys SEO & GEO 360° Audit

To frame an international rollout, a structured diagnostic helps you avoid piling up low-impact fixes. The Incremys SEO & GEO 360° audit helps identify technical gaps (indexing, canonicals, hreflang, performance), semantic gaps (market-level intent), and competitive gaps, so you can prioritise an actionable roadmap before scaling production.

Planning, Producing, and Optimising at Scale: From Strategy to ROI Tracking, Market by Market

Incremys is a B2B SaaS platform for SEO and GEO optimisation powered by personalised AI. It helps teams analyse, plan, and improve visibility across search engines and LLMs, identify keyword opportunities, generate content briefs and an editorial plan, produce content (AI/automation), and track ranking changes and ROI by market through centralised reporting (including the SEO & GEO audit module).

Multilingual SEO FAQ

What is multilingual SEO, and why is it important in 2026?

Multilingual SEO is the practice of optimising a website for multiple languages and/or countries so it can be understood, indexed, and rank well in each market. It matters in 2026 because users prefer content in their own language (CSA Research, cited by Acolad) and SERP competition concentrates clicks in the top 3 (SEO.com, 2026), whilst AI is changing how visibility is achieved.

What impact does a multilingual website have on a site's search rankings?

A multilingual site can increase traffic, broaden your audience, and support international backlink acquisition (SEO.fr), provided you follow specific rules: clean architecture, hreflang, cross-language internal linking, strong editorial quality, and no duplication.

How do you roll out an effective strategy without creating duplicate content?

Use a clear URL structure (subfolders/subdomains/ccTLDs), consistent canonical tags, and correct hreflang. Above all, genuinely localise content (intent, proof points, vocabulary) rather than publishing near-identical translations.

Which best practices should you prioritise when launching a new language?

Start with key pages (home, offer/product pages, high-traffic pages), localise keywords, translate metadata too (titles, descriptions, slugs, alt text), verify hreflang and the sitemap, then measure before scaling up (WPML).

What are the most common mistakes (hreflang, canonicals, redirects)?

The most frequent issues are: non-reciprocal hreflang or hreflang pointing to redirected URLs, inconsistencies between hreflang and canonical tags, overly aggressive geo-redirects, and partially translated pages (SEO.fr, WPML).

Which tools should you use in 2026 to manage international SEO?

For demand and competition: Keyword Planner, Semrush, Ahrefs (Version Internationale). For performance management: Google Search Console and GA4, segmented by country/language. For technical checks: crawling plus validation of sitemaps/hreflang/canonicals.

How do you measure results by language and country, all the way to ROI?

Track market-level KPIs (impressions, rankings, organic traffic, conversions, revenue), segment in GSC and GA4, then connect costs (production, localisation, development) to gains to manage ROI comparably across countries (see SEO ROI).