i18n standards
Locales
Facebook uses a xx_XX format for representing locales like: en_US, jp_JP, etc. We're actively working on separating our language + country combinations internally, and where we go from there as far as standards go is unknown. BUT if you'd like to help support bcp-47 standards or similar, you are very welcome to contribute!
CLDR
We generate all our number variation data found in our IntlNumberTypes internals from CLDR (Unicode Common Locale Data Repository).