The story of The Little Mermaid is of Danish origin, so Ariel would probably sing in that language. In this clever video by Movie Munchies, she switches mid-song from English to Danish. She's followed by Mulan in Chinese, Elsa in Norwegian, Jasmine in Arabic, and Rapunzel in German.
Here's the second video in the series. It features Belle in French, Snow White in German, Megara in Greek, Aurora in French, and Anna in Norwegian. Notably absent are Thor in Swedish and Leia in Basic. Hopefully they will be in a third video.
-via Laughing Squid