Souw, I set my 3DS to englisch the second I got my hands on WarioWare Gold, both because of Martinet and because I figured that the english dub in general would be the most ''relevant'' one to discuss, but now I finally listened into the german dub (german speaker right here).
On the whole it's pretty solid, 9-Volt sounds kind of unmotivated in how he's dubbed and german Wario while not a bad voice in it's own right, doesn't even try to sound anything like Martinet. For the most part it's fine though, Jimmy and Dr Crygor in particular are excellent even and that's not even going into Dribble and Spitz:
This won't mean anything to anyone who doesn't speak german, but those bavarian and berlinerisch dialects not only work perfectly on them, but are just hillarious to listen to, I was laughing myself stupid all the way .