Megadeth's 1994 hit "A Tout Le Monde" was written for Dave Mustaine's mother, who had passed away, and the song served as a loving memoriam from Dave to her. As you've probably guessed, or heard by now, the song's entire chorus is in French.
Though according to Mustaine in an interview with WRIF, it wasn't always that way. Mustaine says French and Spanish were easy enough, but German and Japanese didn't quite go so well.
"We tried it out in a bunch of different languages – some were good, some were pretty horrific. I remember trying – French and Spanish were easy, but we tried two other languages, German and Japanese. After trying both and having both of them fail just abysmally, we ended up sticking with French."
I'd imagine that probably has something to do with the very angular and sharp natures of the latter two languages, as opposed to the easy pronounceability of the former two. Though "A Todo El Mundo" has a ring to it, right?