"Who all is singing?"

Translation: Sparossa vāedis?

[Joel__W]

Why is the English sentence simply not "Who are singing?". That can only be interpreted as plural. I've never heard "who all" before.

[MadLatinist]

"Who" is always treated as singular. In colloquial English you can insert an "all" to indicate it is plural (though to be honest that didn't really occur to me until I saw dedalvs using it in the course.)