Translation:The couple from Helsinki is at home watching an American series.
You're right that it sounds more natural, but not for that reason. A rule of thumb about adverbials in English is that they should be put at the end of the sentence, manner first, then place, then time. "At home" is an adverbial and therefore there is a preference to place it at the end. Also, "the couple" is singular, so the verb should be "is".