i think that's an issue with renpy not reinitialising language settings upon load. (you'll find the same issue if you play around with languages and try to load a language with font A from a language with font B, where A and B don't support the same characters.)
it shouldn't be an issue if you start the game normally after changing the language & have the sound caption init as included in the sample project's start label. for safety you can add the init to renpy's after load label too.
Seems like it's a better idea to tie it to a persistent value and have separate sets of captions for each language, rather than let Renpy's translation function do the work, the captiontool in my game is very buggy regarding the translation function.
I'm taking a look into this myself and trying to find a way, but I'd like your word on it, thanks :D
textbutton "English" action Language(None), SetField(persistent, "lang", "english")
textbutton "日本語" action Language("japanese"), SetField(persistent, "lang", "japanese")