More can always be said on language acquisition, so if anyone would like to address this matter on their own, please don't let this post stop you! For the meantime, here are some previous posts that cover this matter.

u/WelfOnAShelf has a collection of their answers regarding language acquisition, those being these three threads:

• I just read an article on wikipedia that said that the papal states came into contact with the mongols in the middle ages and they sent letters to each other, how could they translate the letters back then?;
• How powerful were translators during ancient, medieval or early modern times?;
• and How would someone learn a foreign language in High Middle Ages?.

Saving me the trouble, u/myfriendscallmethor also has a language acquisition compilation, with u/Iphikrates providing an important supplementary note downthread. This supplementary note is important to keep in mind. Note that, to get to Japan, the first European visitors had to go through literally the entire rest of the world that lay in between Europe and Japan. Lots of room covered by other languages that can bridge the gap.