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I 100% believe that Joseph Joestar is multilingual.
Dude was able to speak with n@zi's who probably didn't speak English all to well, was able to talk to jotaro and kakyuin (who probably didn't speak English all to well), AND he probably learned Italian for suzie q.
So in conclusion, Joseph Joestar can speak AT LEAST four languages. Them being: English, German, Japanese and Italian.