Erica, we run games in Spanish not only because the GM has more vocabulary, it also has to do with some players in the group that wouldn't handle the language, so they would feel excluded. Normally, it's only the GM the one that has enough English in him to handle some translations. But mostly we run the games in Spanish because it's our native tongue, so it would be silly to do it any other way.
When you get to a point in a foreign language that you can also think in that language with no translation required, translation gets easier, because you are not translating word by word, you are translating word and meaning, trying to maintain the spirit in the original phrase. It's also funny that when you get to that point sometimes you struggle to find a word that defines a thing perfectly in one of the languages, because your know there's one, and finally discover that yeah, there's one, but it's in the other language.