"Their third child is a girl again."
Translation:Jejich třetí dítě je zase děvče.
The emphasis is usually at the end of the sentence. In this case, "zase" at the end would mean that "their third child is a girl - Again". It is not possible to the "the third child" twice. I am not a linguist, but this is how I understand it. Moderators can explain it better, I guess.
I'm a native AmE speaker, and I don't find the sentence particularly unnatural (and the Czech translation is quite direct). But I would say that "also" is probably used more often on the English side, and "... is also a girl" is accepted in the reverse exercise. As a bonus, we now know how the thought is phrased in Czech.