I've got a major pet peeve about foreign languages in fiction as well, but for a different reason.
The time it bothers me is when a character (let's say he's french) is able to deliver a long, empassioned speech about the nature of reality without any problem, but throughout the entire book, he'll answer any yes/no questions with 'oui' or 'non'.
This is probably done by the author to remind us of the character's nationality, and probably to make him more distinct, but it has the knock-on effect of making him seem like he's either extremely pretentious or a moron, due to the fact that he can learn the English word for, say, 'Quantum Velocity Cyntrascope' but not for 'yes'.
To be fair, I've found this is most common amongst thriller writers (yes, you there, Mr Brown. I can see you trying to sneak out) though I still see it from time to time in SF.
My other major Pet Peeve, and this is pretty petantic I expect, is the constant mis-use of the term 'Solar System'. This usually occurs when you'll have a group of explorers rocketing off into unknown space and comment about arriving in this or that 'solar system' which they aren't, unless they're back home (Solar System, as I'm sure you all know being the name for the particular *Star System* that we live in).
It's not a serious problem, but I see it appearing an awfull lot all over the place.
I may one day calm down enough to talk at any length about Present-Tense Narration without trying to throw things at the monitor
Simon Painter
Shropshire, UK