Is (Icelandic) vandalism
Is this a regular occurence or just a one-off? In assuming good faith, I would believe it's just an honest mistake. I could see myself doing the same thing – thinking I'm copypasting in one thing, then not noticing I copied the wrong thing. It can happen.
It is the worst edit of several bad ones (of those I have found, anyway). It has also happened that I have fixed his translations and then he just re-adds the old translation, that either can´t be considered an translation or contains a word that does not exist in the language.
It would be better if the user had normal translator rights only. An translator can in theory use their offline translator rights so that they use an old translating file and update from that. The problem with that is that that user would end up ignoring other peoples edits while doing so. I suspect that he has uploaded the same offline translator file more than once, and ignored any improvements by others in the proccess. So, I belive he would be an better translator with normal Translator rights only, as it would show him the most recent improvements every time.
It does not help matters either that there is an lack of vandalism tools here. There is no AbuseFilter, TitleBlacklist and so on.
@Snævar: OK, fair enough. But have you tried talking to him to tell about the problem? I can't see anything in the user talk page, but maybe you have some other communication channels. In any case I'm hesitant to call it vandalism, it seems more a case of not being careful enough. If you want I could post something on his talk page, but it might be better if you do it since you know the specific issues much better than I do, obviously.