AfD or not, serving in a foreign army is a felony in German criminal law.

I’m unaware of any law prohibiting that. IIRC, it’s illegal to be a mercenary. But serving in a foreign military under the same terms as their own professional soldiers is legal.

This article from the beginning of the war said so as well: https://www.lto.de/recht/hintergruende/h/freiwillige-ukraine-strafbarkeit-kriegsgefangene-internationale-legion