I think there are two things being conflated here.
A model may be able to translate content into many languages, but this language selector changes the user’s Discourse locale, including both the user interface and user content.
For example, our AI feature may be able to translate a post into Hindi, but Discourse’s buttons, menus, and labels would remain untranslated because Hindi is simply not currently a supported Discourse locale. The selector changes the language of the whole site, not just the post content.
The options are limited by design to locales supported by Discourse’s i18n system, with the appropriate language labels, interface translations, pluralization rules, and fallbacks.
You can create a plugin - it can register an additional locale, but you would need to provide the proper locale support for all the buttons and labels through i18n, not our AI translations feature.