I believe that this issue was caused by a change to TheMovieDb site (where YAMJ gets most of the covers from). They updated to allow different cover languages for the films and their old API wasn't updated to reflect this, hence why you get covers out of your language.
I have updated the API in YAMJ to the latest one for TheMovieDb and it should now work if you specify the language. If it does not, one of two things could be wrong.
1) The poster does not have a language set for it on TheMovieDb or is wrongly tagged. In which case, create an account for TheMovieDb (its free) and update the information. It's the only way it will get better!
2) I've done something wrong
In which case I will fix it.