Hey! I think Politics would go very well with History and Economics and even a bit of Psychology! Having an a-level in a language is quite impressive. However, unless you want to specialise in something that's got to do with a German a-level , like a translator for instance, I don't think it's necessary. I think Politics would go well with your possible courses in University! You could also decipher between German and Politics A-level by researching the specification for each a-level. After you've done that, see which a-level seems more appealing because at the end of the day you're going to be doing this course for 2 years and it definitely needs to be something you enjoy or you'll just lose motivation and probably get a poor grade.
I hope this helped! :-)