I assume you mean bands that not only come from germany, but who also sing in german, right? (Otherwise it would be quite pointless for learning the language ^^)
Well, the standard-answer to such a question
, Rammstein is of couse always a good start.
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Probably one of Germanys most internationally well-known bands. If you like them, you may also like
Oomph! - not quite as well known, but were actually one of Rammstein's main sources of inspiration.
A good german-singing metal band imo is
Equilibrium - good luck understanding any texts though, I sure as hell can't most of the time! ;D
Eisregen are a little bit easier to understand, but not much.
If you like it a bit softer,
Subway To Sally might be for you. They are kinda soft metal/ folk inspired/ hard to classify...
Die apokalyptischen Reiter are also quite good
. Knorkator are always a lot of fun to watch and especially amazing live - they make awesome metal-parodies and other fun stuff and are just generally crazy. (Only funny if you already understand the texts
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).
Other bands that come randomly to mind (not necessarily Rock/ Metal...too lazy to look up the exact genre now):
Megaherz (be a bit careful with using the words you learn through their music in class though
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),
Das Ich (Whatever genre that is...they call themselves "Neue deutsche Todeskunst". Inspired by Gottfried Benn's poems.) and
In Extremo. (Medieval Rock. Or at least they were Medieval Rock last time I checked. Nowadays they seem to be more into...steampunk? Not sure, haven't followed them for a while now.)
Edit: Okay, one more. Not to be confused with Eisregen:
Eisbrecher