Thank you for your suggestion. It is a possible translation, but as a native speaker, I don't find it a very likely one. It is much more likely that if you tried to book a table and the restaurant was complet, you would be told it was full or fully booked. We didn't have fully booked among our translations, so I have added that. Unfortunately, because there are a number of ways of saying complet in English, I don't have enough space to add at capacity, and I think it is less useful/common than the other translations we have. Therefore I won't be adding it.