Hi, Dominik. I've never watched this cartoon but I checked up on Internet and I'm sure IT'S CHINESE. These four words are "降世神通" (jiang4 shi4 shen2 tong1) in simplified Chinese. "降世" means gods or superpowered guys who belong to outer world come to earth. For example, Superman is from Krypton. We can call his coming to earth "降世". "神通“ means someone with super power or gifts, just like superman. So, "降世神通" means someone with superpower came to earth.

Whatever it is, it is Japanese.

It's the direct translation of "Avatar" into Chinese (I mean Aang, not the blue guy from the sci-fi film Avatar)! When you watch the show with Mandarin dub, you can hear this word there a lot :)

Incarnation (as opposed to apotheosis) of a God (supernatural power)