If your German is rusty, it's like the bike you left out in the rain too many times that turned brown and flakey on the metal parts. Like your bike, your German skills became rusty, meaning they're suffering from neglect.
You can also describe yourself as rusty: you used to do flips on your skateboard, but you haven't practiced in a while and now you're a little rusty. Rusty can also refer to the color of rust, which is a reddish-brown — a rusty-colored dog, for instance. Anything made of iron or steel that you leave outside in the elements is likely to oxidize and become rusty, covered with rust.