How do you say in English, "to eat at a restaurant" or "to eat in a restaurant"?
Normally you would say:
eat at a restaurant.
This would imply that you went to the restaurant, were served food, and ate it. If you ate your own food (or food from another restaurant), you would have to explain that.
If you were to say:
eat in a restaurant
That would mean you ate food while inside the building.
While both could be used, at implies being a customer, whereas in simply indicates location.