Another metalhead here, and I listen to songs in:
English (English isn't my mothertongue) (for example Within Temptation, My Dying Bride)
German (Rammstein, Knorkator)
Russian (Arkona)
Swedish (Vintersorg)
Finnish (some songs by bands which mostly sing in English)
Arabic (Myrath)
Yiddish (Orphaned Land)
French (Arkan)
Portugese (Moonspell)
Turkish (Almora)
Sumerian (Therion)
Gaulish (Eluveitie)
And once in a while also to:
Chinese
Spanish
Italian
Greek
Japanese
Gaelic
Latin
Edit: Thought Arkan had French songs as well, but although they speak French in between songs the songs are (at least mostly) in English:
http://www.youtube.com/watch?v=8Kf7lSo-v34
Maybe I'll think of another one instead later