1. Targeting fbt (Meta's internal i18n tool)
2. Used CST (<3 ast-grep) instead of AST - really useful here IMO esp. for any heuristic-based checks.
3. Fun fact: this was made entirely on my phone (~2.5h) while I was walking around Tokyo. Voice prompting + o1-pro. Why? My friend was working on porting fbt to TS and said he was planning to build this. I wanted to one-up him + convince him to start using LLMs =)
One thing you should be aware of is that for at least Japanese, localization is far from just translating the text. There are lots and lots of Japan-specific cultural nuances you have to take into account for web users and even down to actually just having an entirely different design for your landing page often because those you'll find those just convert better when you know certain things are done that are typically not done for you know non-Japan websites.
Notta (multi-lingual meeting transcriptions + reports) is a great example if you compare their Japanese [1] and English [2] landing pages.
Note how drastically different the landing pages are. Furthermore, even linguistically, Japanese remains a challenge for proper context-dependent interpretation. Gemini 2.5 actually likely performs best for this thanks to Shane Gu [3], who's put in tons of work into having it perform well for Japanese (as well as other "tough" languages)
[0] https://github.com/f8n-ai/fbtee-migrate
[1] https://www.notta.ai (Japanese version)
[2] https://www.notta.ai/en (English version)