Another part of it is the times in which they're translated. There are a lot of English lyrics to Welsh songs which are terrible, cloying, Victorian words to sometimes quite interesting Welsh lyrics.
Another aspect of this is ethnomusicologists who study songs in translation and come up with some rather off-base theories. This came up recently with my friend who's research Wren songs (another thread). She says there are some strange theories floating around and you can see where they come from - bad translations.