My biggest beef with yaoi is how the yaoi fans seem to be the same people who are fans of inserting Japanese into their fics.
Now, that in itself isn't too much of a problem as long as
1. They have a DECENT grasp of what kind of characters use what kind of Japanese (for instance, a big manly tough guy isn't going to say "Hai" or "Gomen nasai")
2. The characters are actually Japanese (this means few, if any, characters from Gundam Wing). This can be ignored if the story is actually WRITTEN in Japanese.
3. They don't just randomly insert Japanese words into English sentences ("Oh, Heero, you're so kawaii")
Also, it's a little more believable if they don't use words and phrases that EVERY fan of anime knows ("baka," "arigatou," "sushi").
That goes for any language, really. I know I'm focusing mainly on anime here, but unfortunately I don't know enough about other fandoms to read, let alone write, their corresponding fanfiction. I'm also quite passionate about foreign languages, but even I don't feel competent enough to go around inserting Japanese phrases into my work.