I wanna start writing in English and I have a few stories in Russian. What's my best choice? Is it to rewrite those stories in English with very many mistakes or to start something new and learn?
Writing in a language other than your native one is hard (so is writing in your native language but this is usually harder). On the other hand storytelling is storytelling.
If you are just practicing the question is what are you practicing. If you want to translate then translate, if you want to tell stories then tell a story.
If you want a saleable story you can either hire a translator (expensive), do your own translation (hard), Tell a well written story (hard), or tell a poorly written but well told story.
When deciding between the last two the question is do you want to hide your foreign language roots or exaggerate them and the answer will depend on the story you want to tell.
If you are comfortable with your storytelling skills and quality of your writing in your native language, but you want to expand your language horizon, I would suggest first to try translating a story you already have finished and consider well-written. This way you can concentrate on the translation and not the plot/characters, etc.
As for should you do it or not, it is for you and you only to decide, and for the time to tell.
I can only praise you for challenging yourself.