Oldboy, if I understand you right you think that something is wrong with cepo's sentences, but there isn't. They are absolutely valid English utterances, and the "does" adds emphasis to its verb phrase. "She does speak English" highlights the fact that she is able to speak English, probably as a second language, if not Russian, whereas the other sentence simply contrasts being able to speak the one language but not the other.

However, your post does answer (see, I'm doing it, too! "Does answer" adds emphasis.) the original question inadvertently. Your example ""По-английски она не говорит, а вот по-русски умеет." achieves what the added "does" achieves by adding the particle вот. Such particles are a way to add emphasis or slant the meaning of an utterance in a specific direction.