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.
Judging by the quality of most of the foreign stuff I’ve heard the original voice actors do them.
Reminds me of Nodame Cantabile (not a cartoon). It’s a Japanese series where later on a German secret agent comes to Japan. It’s clearly a Japanese actress in a blonde wig. And she briefly talks to the canonically Japanese student in German. His German is soooo much better than hers. His is almost flawless and hers is atrocious. But she is supposed to be the native speaker.