"We are going to a restaurant in the square."
Translation:Wij gaan naar een restaurant op het plein.
"Naar" is used when going towards something or to somewhere usually
"Op" I have noticed used when talking about a physical location. "I am at the restaurant," being "Ik ben op het restaurant." I tend to think about it as "I am on the restaurant" first before translating "op" to "at."
And "Aan" I think is used for things that aren't physical, such as thoughts, ideas, languages, etc. I still think of it as "on" before translating it into another word. For example, "ik denk aan haar" I comprehend as "I think on her" before translating it to "I think about her."
These are just things I've been going by as I've been learning. I'm not sure if these are correct or not, so I would check with a grammar site before just taking my word for it
You could say 'Wij gaan op restaurant op het plein'. 'In het plein' is never correct.
But there is a slight difference between saying 'Wij gaan op restaurant' en 'Wij gaan naar een restaurant'. In the first case it is 100% sure you are going to have a meal in the restaurant.
'Op restaurant gaan' means 'going to have a meal in a restaurant.
'Naar een restaurant gaan': most of the time you also want to say you are going to have a meal. But it also could mean you are going to the restaurant because of another reason (delivering something, picking somebody up, ...).
You are right, that is 'Belgian dutch: