Honestly, I'm not sure that's the case. I've met some people that almost certainly wouldn't understand Japanese and yet I haven't had any problems. I do speak a few other languages, but it doesn't seem like finding a common one is necessary.

[It's not too out of the ordinary for her since everyone in Chaldea understands each other, whether they already speak the language or magic's involved. Though it is a bit strange to have it happen without being aware of it. She has so many questions for who or whatever's behind all this!]

I imagine it'd be hard for us to fit in if we didn't understand each other. At least, that's the only line of thinking that makes sense to me.