tubtouch, I neither read Japanese, Chinese, Russian, nor German (there's more but let's keep it short), but I know how to tell when things have a different name in those respectful languages.
It's simple, he's not looking at the name of the quest mob vs. the name of the mob he's attacking careful enough. And if he's been there for hours, sadly he doesn't have the logic skills to go "Wait, something is wrong. Let me ask for help" which does not matter what language you're familiar with. In my server alone, I've seen people speak Russian, German, Spanish, and a few other languages. Someone on your server may know his native language, but he's not trying to use that to his advantage.
If he's a friend of yours, you need to teach him these things. Tell him to take his time looking at the names. Don't be afraid to ask for help in his native language if he can't speak English all too well. Sure there will be douchebags who may respond but there may be a person wiling to help who speaks the same language. Don't look down on the power of using your brains!