Learning a new language is always hard, but the people of the past were able to and the people of now are, so it shouldn't be too hard for him to just point at things and say the words for them, and eventually she will be able to know what "meat" is in Japanese, and the process could go in reverse, so he would learn her language too, but grammar might be a problem. Either way, I don't think that people who spend this much time together would not be able to learn each others language in a timely manner.