How to maintain a language
I am learning Russian and think I can complete the tree in approximately 10 days, after this I want to learn German. I am really excited to learn and wanted to start now but think it is best to start after completing the Russian tree. I was wondering after completing the tree how is it best to maintain my russian while focusing on another language?
You could do the German for Russian speakers course. This is called laddering.
Talk to native Russian speakers regularly. If you can memorise all that vocabulary and grammar in ten days (which is a miraculous feat in itself), it's the best way to keep all of it in your head and increase your ability.
In fact, conversation is the best way to maintain your ability in any given language full stop.
one good way is to continue to strive to keep your strength bars full. You could also have a notebook in which you write down words or concepts that are difficult for you. Another good way to keep up with the language is to find books (children's books would probably work best) that are in Russian to practice reading and understanding grammar concepts better. Good Luck with you tree! I hope you have fun learning.
Do Russian and German in different times of the day so it's easier to associate and not get mixed up eg Russian in the morning, German in the evening
Use flashcards once in a while to remember your vocabulary/grammar, etc
Write stories in each language once in a while
Read text in your languages
Consolidate your listening and speaking by listening to podcasts, videos, radio, music lyrics, speaking with natives etc