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.
Mostly they would use the original voice actor. It’s more of a hassle to find and recruit talent that sounds similar and add them to a payroll, add a line in the credits, schedule voice recording times etc than getting the original to learn a paragraph in a foreign language. It’s not hard , like you said, but it’s a hassle and it takes time away from several people in different areas.
You would have exceptions of course, perhaps the show has very eccentric producers/directors and enough of a budget but when it comes to resource management a couple minutes of voice dialogue usually isn’t worth it.