First, are we all in agrreance that this pretty much confirms Yoshi's return? It seems kinda like it should, despite the fact that Yoshi hasn't been actually specified as a confirmed character.
Secondly of all, the yoshis aren't saying anything in the **** song. The yoshis do not speak English, (unless you count them saying "Yoshi!" which probably shouldn't count) so why would that suddenly change for a song? It seems to me, that, as childosengan put it, they are just saying "EEE AAAA OOOO."
Finally, are we sure that this is Yoshi's stage music? I mean, I know its the "Yoshi Story Ending Theme" but it sounds like it would be well suited for some sort of music for Yoshi's ending after you beat the game as him. I wouldn't guess its his victory fanfare, but that could be debated I suppose.