You can add your custom languages to the game using the LanguageService.RegisterLanguageCode
method.
LanguageCode code = LanguageService.RegisterLanguageCode("japanese");
There hasn't been much progress on actually implementing custom languages. No one has ever translated the entire game to a different language, not yet included with SoR. When that time comes, feel free to create an issue on GitHub.
LanguageService
LanguageService
is a static class, that contains all things related to localization.
TODO: Waiting for custom languages implementation in RogueLibs
Eh. Just ask the creator(s) of the mod, that you want to translate, to include your localization.
TODO: Waiting for custom languages implementation in RogueLibs