In practice a programmer who really, honestly, and absolutely knows only a single language is bound to be exceedingly rare. If you think about it (and possibly include meta-languages) you'll find that the vast majority know at least two, even if they're not fully aware of this fact.

But do they really know the language, or are they merely able to use some of the features (just like you learn common phrases of natural languages, but don't understand why are they built in such a way)?

I don't think I can say I really know English, tbh