This. I mean I have seen a few instances (more in published books than fanfic) where people said something and then either the narration or the dialogue afterwards made clear what had been said but that's really not that easy to pull off. (I once read a book which had a weird half/half thing. It had a glossary at the end that translated some words and some were made clear from what came afterwards. Only there was no indication what was what so sometimes a word came up, I checked the glossary, cursed because I didn't find it, read on and discovered that a paragraph later the context made clear what had been said).

Plus as you said there is rarely a point to it. The book I just mentioned admittedly had one (a crime novel set in the 19th century and the small-time criminals were sometimes talking in Flash (a special 'thief-speech' of the time) and while I wasn't a fan of the way it was integrated it did add to the atmosphere. But just having two random characters talk in Spanish/Russian/whatever because the author knows the language doesn't add that much to the atmosphere.