These captions are set on a platform level - ie not for individual apps but for REI3 itself.
Officially we support "de_de" & "en_us", but many translations have been provided by community members - these can be viewed here: https://github.com/r3-team/r3/tree/main/www/langs
If your language is not included or the translation is incomplete, you can, if you want, update existing or create new language files. If the quality is high enough, we could then, with your permission, include these translations in the next patch release of REI3 - then you and everyone else would have access to them.
