Nevermind, I think I found out.

In episode five of Cowboy Bebop, during the violent climax, is a very bright tune sung by a children's choir. While the latter song resembles a hymn sung in Latin, it's actually just nonsense words made up by the artist, Yoko Kanno.

The choir they're referring to is the same used in "Blue", so I assume it is the language created by Yoko Kanno. She uses it in other works of hers, but refuses to provide an interpretation for listeners.