No problem. I'm assuming you're in Europe - most of their console games are multilingual, but DS games, you hafta be careful. Ubisoft are always at least English/French (usually Italian, Spanish, and German, too), and Zelda are at least English/French wherever you go. Not sure about specifics on others, but be careful in general.
Hopefully you got that guy's name and can point him out when returning it, most places don't let you return games (at least, in Canada) that have been opened. You'll probably hafta show a receipt, packaging, and clarify who lied and sold you a game that was supposedly multilingual, but actually wasn't.