I'm working on novelizing the story of Joseph from Genesis, so it got pretty hairy in places language-wise.
For instance, when he came to Egypt and didn't understand a word, I ended up with an entire chapter with no dialogue. I was careful to talk about the sounds of the language (via description, nothing in quotes) but make it clear that Joseph had no clue what was said. As he learned the language I put full Egyptian sentences in quotes since people tend to understand more than they speak. But I was careful to have his responses sound stilted, and describe his frustration with not having the correct words.
At the end when his brothers came back, I had to jump between Egyptian and Canaanite pretty frequently. I was just always careful to have which language was spoken be in the dialogue tag, as often as necessary.
Anyway, that's how I did it!