I definitely do not have a problem differentiating English from Japanese.

Now, this might actually be the case for languages like Spanish, Italian and Portuguese. They are so similar, you could sometimes just infer what a word in one language is if you know it in another (heck, I can do this with a lot of languages I don’t know just because they all have the same root language, so many words are just a letter or two in difference). Where I live, Spanish and Portuguese speakers are pretty common, and they can usually talk to each other not even knowing the other’s language because they’re so similar.