I'd guess maybe 15-20% of all the music I listen to is foreign language. Some favourites that sprint to mind are Manigance (French), Saber Tiger (both English and Japanese songs), Mago de Oz (Spanish), Aspid (Russian), Moonsorrow (Finnish) and Melt Banana (Japanese). Some genres in my collection are very heavily foreign language (eg Latin, flamenco etc mostly in Spanish or Portuguese, Viking/folk/etc metal usually in a Scandinavian language). Hell, Trollfest even 'invented' their own language Trollspråk which nearly all of their songs are in, and is supposedly loosely based on a bastardisation of Norwegian and German.
Given that a lot of metal bands have vocals which no-one could possibly understand without a stand-alone lyric guide, I think it's one of the easier genres for bands to reach an audience outside of the native speakers of the language they're singing in.