I agree with what was said about the marketing factor, but I wish that more bands would write more lyrics in their native language. I'd love to hear Behemoth writing in Polish again, or Deathspell Omega singing in French (or Latin/Greek, since it's appropriate).
That was what made Rammstein so appealing to me when I was first getting into metal; the fact that they sung in German almost exclusively made the band unique in the mainstream.
And singing in a native language, to me, makes the message/delivery seem more meaningful and emotional, such as Hellveto singing Polish or Taake in Norwegian. If I may quote Rammstein's lead singer, they sing in German because "German is the language of anger."