I am absolutely sick of people using foreign languages in their fanfics. In my opinion, they shouldn't be used at all unless it's a special situation or you're talking about something that has no good translation in English, such as some Italian dish or a street name in Romania. If you absolutely must make Germany and Prussia call each other "Bruder" or something like that, okay, fine. I can deal with it. Same for a couple of French endearments from France etc. Those can work.
What I can't deal with is when people use entire phrases of foreign languages in their fics. More often than not, they used the Google translator for these phrases. As a result, they're full of grammatical errors or the vocabulary doesn't match the context. Never ever use an online translator. If you don't speak the language yourself, ask someone who does for help.
My second issue with foreign languages is that they're incredibly jarring, even if the lines are correct. If I'm reading a fic, am suddenly confronted with a line in Russian and have to scroll down to the bottom of the fic for a translation and then back up to where I was, it kicks me out of the story and ruins the reading experience. Making the characters speak in their native languages doesn't make them more in character or give the story a French/Japanese/Polish/etc. flavour. It's just bad writing if it's done without purpose.
As an example, I just saw a story in which half of Prussia's dialogue was in German, Russia's in Russian and France's in French. Why? Why would anyone think this is a good idea?
Do authors even stop to think about what they're doing? Most people don't use foreign languages the way Hetalia characters do in fics. Most people don't go around speaking otherwise perfect English and then adding words like "yes", "no", "sorry", "excuse me" and so on in their native language. I spent last year as an exchange student in Germany and got to know students from all over the world. NOBODY inserted words or phrases of their native language into their speech, even when they were having problems expressing themselves in German. The reasons are that
a) the kind of words I listed are the first you learn in a foreign language, so you don't need to refer to your native language when using them
b) using your native language is a sucky move when the people you're talking to don't understand it
Also, the students from Japan never used honorifics when they weren't talking to each other in Japanese.
I'm not saying that using foreign languages is always bad. There are certainly situations in which it's appropriate, such as when a character is so surprised that they automatically revert back to their native language. However, more often than not I see lines upon lines of completely mangled language X in a fic, and it looks like the author only put them there because it looks cool. In my opinion, the use of foreign languages should always serve a purpose, and the author should be careful to make sure that what they're writing is correct.