I want to start focusing on my German now I have completed my Russian tree, I had a lot of time at the end so finished the Russian tree quickly, however I cannot focus now on German because my Russian skills are deteriorating to quick so when I go on I have 20/25 skills to make gold again.
Is there anyway I can slow the skills from deteriorating so I can focus on German?
Not really. They will decay more slowly as you practice them over time. Space repetition can be annoying like that.
If it bothers you, by all means spend some time practicing old skills. Otherwise, don't stress and just move on to German and practice Russian when you feel like it. There's no special need to keep things golden.