Think this varies greatly on the character you’re encountering. Don’t really think it’s far fetched for numerous concepts/characters to have at least picked up on phrases or the like. Insults especially. I don’t really speak a lick of german or greek but i know how to insult/curse at someone in them due to osmosis from people i’ve work/ed with.
Honestly, just take the gamble and write out the language filter. If someone is a tool about it and clearly being a dork over roleplay then just walk away.
I think the opportunity for enhancing roleplay by letting characters learn languages far outweighs the potential abuse.
I have a troll who has worked and lived among goblins so long he has entirely adopted their culture. It makes sense that he should be able to speak their language. And he does, IC. When goblins speak in [goblin] and I can’t understand, I tend to just handwave it as them speaking too fast for him, since it’s a second/third language for him, but it would improve my roleplay if I could speak it.
I also have a character who works in spycraft, and he has naturally learned some of the languages that are relevant for his field.
People who would abuse this function by nonsensically knowing every language without reason, are probably a tiny minority who do this already, albeit in more roundabout ways.
I don’t think OOC being difficult with random people depends on the character?
Or if you mean language knowledge, then I agree, though it’s worth noting that typing it out is still an optional courtesy, you’d go way farther being nice and throwing a whisper like “hey, would you mind letting me know what your character is saying? Mine knows the language” rather than being passive aggressive in /e or whatever.