I think for a band to sing in their native language lends so much more character to their music, makes it more interesting.
I read once in an interview of Timo Kotipelto when he was asked if Stratovarius would ever make a song with Finnish lyrics - his response was that if we were to hear a Stratovarius song sung in Finnish that we would know why they won't do a song like that. I wonder if he has even tried it.
He has obviously not heard Moonsorrow. I would be very disappointed should Moonsorrow ever choose to sing in English. That band has the most beautiful blend of musical elements. There's some very talented people in that band.