no-one has tried to speak german to/with me (as the pretty much only commonly known words are sauerkraut, kindergarten, über, und gesundheit), so i cannot say, but why would i?
i think it's far more offensive when people switch to something different than english and (try to) insult you in their own-but-not-your language. happens quite often with french, at least in LoL; then again, MOBA "communities" are per definitionem a conglomerate of first-class [censored], so i guess that doesn't mean much
although it's quite interesting to tell them, after a couple of minutes, in french that you actually do understand them. maybe insult them in turn, just to prove you can (if you're in the mood), and just for fun