Usually, "does" or "did" can be translated "действительно" or "на самом деле" or "и вправду". For example "She does speak Russian" - "Она действительно говорит по-русски" or "Она и вправду говорит по-русски".

In your example I think it's better to place emphasis with your voice and word order: "She doesn't speak English but she does speak Russian." - "Она не говорит по-английски, но по-русски она разговаривает." (you raise your voice a little on "по-русски" to show emphasis). I think you can use italic or bold in written text to place emphasis. Also, I changed the verb from "говорит" to "разговаривает" to avoid repetition of the same word.