Here's something for you to try if you enjoy extreme methods in language learning. Learn a language without any materials from your native language or any other language. Don't use a dictionary unless it is a monolingual dictionary in the language you are learning. Don't make any notes either. Keep all the info you learn in your head.
Do you think it would be impossible to learn anything this way? You would have to start out by just guessing at what words mean. Do you think no one could make heads or tails out of the language? But this is what Michal Ryszard Wojcik did in the Norsk Experiment when he started learning Norwegian in May 2001. He was able to figure out what a lot of words mean just by reading and re-reading in the language. He had learned English and German previously and was able to transfer his knowledge from those two languages to learn Norwegian. According to the how-to-learn-any-language website, there is a 30% overlap of vocabulary between English and Norwegian and a 60% overlap between German and Norwegian.
You should also read his report from another person who learned Norwegian the same way. This person is American and doesn't mention whether or not she has studied German.