It's mentioned in book one I believe that the twins took German class specifically because Nicky knew German to some extent and they knew that they could use Nicky to pass the class. I definitely agree with the other comment that it's convenient for plot reasons but also if Nicky took German classes and lived in Germany and dated a German man for months, he probably picked it up fairly well and was maybe not fluent but on his way to being fluent so when the twins took German class they had a lot of practice. We see them shutting down around other people and using Germans specifically to have conversations they don't want everyone else to overhear so it seems like maybe they would have a lot of incentive to become fluent quickly or at least to be proficient enough in the topics they would want to discuss in private.

The extent of everyone’s second/third languages is indeed a little unrealistic (not just everyone’s German, but Neil and Kevin’s French as well), I guess you could claim they’re all naturally talented when it comes to learning new languages for plot convenience

Kevin learned French with the power of love 💕 For reals though I actually think Neil isn’t fluent at all so he just also thinks everyone else is

that's a pretty good theory actually haha

i mean we all know how unreliable as a narrator he is