It's worth noting that there is at least an unofficial russian language pack (and I saw a german one on msfn once).Currently, no Basilisk language packs exist for any language. I've had a few people ask for some other languages and have offered to help them make one but no one has stepped up to the plate to actually release one.
However, there is a problem with offering it officially: since it was originally a langpack from ff52, which has been consistently improved by many people over the years, you can't be sure that there are no redundant strings and the translation is correct everywhere.
In the case of PM this is solved quite simply - pale-moon-localization repo that has an en-US reference package that can be used as a basis or for translation checking.
This is not the case with Basilisk. Of course, there is /basilisk/locales/en-US in the source code, but it is much less convenient.
So maybe should create a repo with a reference language pack for Basilisk as well? In this case, I think there will be a better chance that someone will translate it into other languages. Just because it will be a basic version, and also can use the text from PM langpacks - as my experience shows, it matches the text from Basilisk by about 90%