"She only speaks seldom."
Translation:Sie spricht nur selten.
Yes, or just "she seldom speaks." Actually, I think "She only seldom speaks" is the more natural English word order here.
It is totally unnatural in English!!!! It's something a foreign person might say mistakenly
Yes--however, I'm sure it sounds natural (in English) to a native speaker of German! It may be one of those "I can't believe this isn't natural" sentences (In this case, for a German speaker, in English). A German example:: "Die Kinder sind mit ihren Grosseltern.", which seems so basic, it ought to "work." However, in German, you need to either use "bei" instead of "mit", or throw in a "zusammen" at the end of the sentence, to sound natural. As second-language learners, we never reach the level of "deep knowledge" about natural language patterns that a life-long speaker has. (Which makes things interesting!)