I don't think "dead" languages are excused from being used as lyrics. If that were the case, then Hughes wouldn't have run into a problem using Carmina Burana b/c the libretti is in Latin. "1492" is acceptable b/c the lyrics are a background to the musical score and aren't clear enough to be distinguished (although I know they are in Latin). I think it also depends on the sound system and the skating venue as to whether or not vocals are noticeable. Watching the men's SP on TV was the first time I had actually heard clear vocals in Joubert's music (and that piece of music was composed for him, so he really should have known better)
The issue with vocal music that has lyrics (and yes, it
is possible to have vocal music w/o lyrics) is that, by not using lyrical music, it makes the skater have to interpret the
music rather than simply acting out w/e the vocalist is singing and having "skating for the deaf." It's easier to allow lyrics in ice dance b/c the dancers have to reflect the rhythmic beat running through the music (and the music has to
have a running beat---IIRC, D/V ran into this problem at '06 Euros)
When I was doing choreography for modern dance, my teacher wouldn't let me use music with lyrics for the same reason (ironically, I ended up using Nino Rota's "R&J"--probably one of the most overused pieces of FS music ever, but hey, it was on the floor and not the ice

)