Most of the GP32 OS is in English (at least if you buy it from Craig at GBAx.com).
As for games, ther are quite a few around in English, though the ones where the language matters most, RPGs, are mostly in Korean. Evben the ones that have been translated are usually done in some hilarious 'All Your Base are belong to us' way too
I just bought PInball Dreams, Little Wizard and Dungeon and Guarder, and these are all in English (though, like I said, the Dungeon and Guarder translations made me chuckle).