I could see the voice actor learning a sentence or two but on occasion the bit goes on for a couple minutes. I feel like with a cartoon it wouldn’t be that hard to find a stand in who sounds close enough or use whoever does the dubbing for that language.
That can totally depend on the situation and who’s the voice actor. In the case of My Life As A Teenage Robot, I’m pretty sure in the episode where Jenny loses her English voice disk, her voice actor does both her English and Japanese form the whole episode. I could be wrong about that’s so feel free to correct me if I am.