Translation:Four boys and many girls sleep.
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I'm not sure about that since in Italian it would be "I miei figli e le mie figlie dormono" although we could say "i miei figli dormono" (which would be more common to say since it's faster), "figli" being masculine like "filii" but it would include both masculine and feminine genders, so my guess is "filii mei dormiunt" would include both genders, and "filii mei" would mean "my children" in general. In Spanish it would be "mis hijos duermen y mis hijas duermen", "mis" not having a gender, but it could also be "mis hijos duermen" same structure as in Italian to include both. In Portuguese it would be "meus filhos e minhas filhas dormem" or "meus filhos dormem" so structurally it wouldn't change much from Spanish. I have no clue about French though. So I think it's either "filii mei et filiae meae dormiunt" or "filii mei dormiunt".
Well I don't know. But this doesn't sound very classical to me. We only used to read and translate anyway and of course there were church music and church texts. I think it was a different tradition, for singing we sometimes changed between Italian/German pronounciation.