If you create a new testuser,
then login to OWA (before using outlook) with a browser which language is set to spanish,
on the OWA login also choose the correct language (spanish),
send/receive one mail.
Your folders now should have the correct language variant.
Now, add this user to the BES.

Does the BES handle the folder names correct or not ?
If not, I guess there is a problem/bug with the BES, if yes, it should be a problem with your exchange.