This post is for Day 8 of Merpay & Mercoin Advent Calendar 2025.For more than a decade, Mercari’s Marketplace has been a

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Mercari Engineering

Mercari Japan launched English UI support across iOS, Android, and web after years of preparation. The team leveraged existing i18n infrastructure built during a previous rebuild, used AI to review 15,000 strings and identify issues, then had human translators fix ~1,000 flagged items. Key challenges included cross-team alignment, QA testing for locale-specific bugs like truncated strings, platform-specific routing and locale handling, and backend microservices localization. Extensive dogfooding revealed issues like mistranslations and address input requirements. Phase 1 covers static UI; Phase 2 will tackle dynamic content and user comments with AI-driven translation.

Finally, Mercari Japan in English! Our Road to Cross-Platform i18n

Getting from localized strings to releasable strings (with AI)

The non-technical side of making strings releasable