If I'm not mistaken you must have the Spanish input method installed -- when you connect your device to the PC, does Desktop Manager list that Spanish is installed? There are two "parts" - one is the input method, the other is Spanish language for all the Blackberry stuff. If you want your device left in English then you only need to install the input method alone.
(I'm going on my Pearl knowledge but I think all BB's are the same regarding this. It's been ages since I actually looked, going on memory here.)