Formal:
* Мой русский язык хорош. (Moy russkiy yazyk horosh.) - This is the most literal translation and is appropriate for formal situations.
* Я хорошо говорю по-русски. (Ya horosho govoryu po-russki.) - This means "I speak Russian well" and is another formal option.
Informal:
* Я неплохо знаю русский. (Ya neploho znayu russkiy.) - This means "I know Russian pretty well" and is a more casual way to say it.
* У меня русский язык неплохой. (U menya russkiy yazyk neplohoy.) - This translates to "My Russian is pretty good" and is also informal.
More specific:
* Я свободно говорю по-русски. (Ya svobodno govoryu po-russki.) - This means "I speak Russian fluently."
* Я хорошо понимаю русский язык. (Ya horosho ponimayu russkiy yazyk.) - This means "I understand Russian well."
Ultimately, the best way to say it depends on the context and your own level of confidence in your Russian skills.