That should have been accepted. If it was a normal translation exercise please use the Report a Problem function to notify us and select that your sentence should be accepted.
If it was a listening exercise only alstjeblieft is accepted since that is what is being said.
While it would be a very specific situation to say 'Signs, please.', that is an acceptable translation in my opinion. Though generally in Dutch you would indicate what kind of sign it is. If you think another translation should be correct just report it.
Plates and signs are not the same objects, I've always determined borden to mean the equivalent of dinner plates. I could be wrong but that's just how I see it :)
In English plates and signs are not the same objects. In Dutch though 'bord' is being used for many things. Here's a definition of the Dutch dictionary:
bord (het; o; meervoud: borden) 1 houten blad bij verschillende spelen: dambord, schaakbord 2 groot plat oppervlak voor opschriften: uithangbord 3 schoolbord 4 ronde plaat of schaal, m.n. om van te eten
So 'bord' can be used as a plate from which you eat, but also for:
traffic sign - verkeersbord
blackboard - schoolbord
license plate - nummerbord
billboard - reclamebord
sign board - uithangbord
notice-board - prikbord
and even to a piece of wood can be referred to as a 'bord'
In all of these case if the context makes clear what you are referring to you can only use 'bord'. So yes, plates and signs are different things but they can both be referred to as a 'bord', along with a few other things like I mentioned.
I guess "plates" are the physical object where you put the "dishes" in order to eat. And "dishes" are more like an abstract concept to define some sort of meal/food/etc. But I can also be wrong...