The only ones I have in German are
"Stadt" by Cassandra Steen,
"Wir Werden uns Wiedersehen" by Selig,
"Hamma!" by Culcha Candela, and
"Deutschland" by Die Prinzen.
The rest of the German artists on my playlist perform mostly in English, including:
"Allein Alene" by Polarkreis 18 (remixed by Danish band Nephew (original version located
here)). To be honest, this is the only song I've heard by this group (Nephew fan, not so much a Polarkreis fan), so the rest of their music could be in German for all I know.
"Dragonfly" by Edguy
"Mr. Torture by Helloween