"When do you want to get married?"
Translation:Kdy se chcete brát?
In this case it is admittedly tricky. JÁ SI BERU TEBE. = I am marrying you.
TY SI BEREŠ MĚ. = You are marrying me. We are taking something for oneselves.
MY SE BEREME. = We are marrying each other. We marry SEBE.
If you said MY SI BEREME... it would look like we are taking something but you did not tell us what that something was. For example. MY SI BEREME ČEPICE. we are taking hats.
First, note that here we have kdy. But kde is also possible if you want to ask where.
Brát si requires an onbject as kacenka already illustrated. You need to "brát si" someone, the object is completely necessary, it means "to marry" .
"Brát se" is a different verb and it means "to get married". "kde se chcete vdát/ženit" is also possible and clearly asks for the opinion of that single person, perhaps the partner has a different plan. "Kde se checte brát." implicitly ask both partners even if just one of them is present.