"Sie spricht nur selten."
"She only speaks seldom" is correct (I prefer "she only seldom speaks"), as is your "She talks only rarely". Both constructs though wouldn't be heard (in North America at least) very often. If I was trying to express the sentement naturally, I wouldn't use "only" at all. "She rarely talks" or "She seldom speaks" both work well. You would most likely hear "she hardly ever talks" the majority of the time. Hope that helps!