ワンオーダー制って英語でなんて言うの？

Most western people are familiar with cafe culture and will order first.

"Seats are for customers only" states specifically that the seats are only to be used by customers who have ordered. "Please place your order at the counter" Politely/kindly/respectfully asks the customer to place his or her order at the counter themselves. "Please place your order at the counter before taking a seat" Politely asks the customer to first order at the counter then find somewhere to sit once they have done so.

This is not a type of restaurant in the UK.. Maybe you would call it a self service or counter-service restaurant. People in the UK do not usually sit down in a restaurant without ordering anything. It is disrespectful to the restaurant owner. In a place like MacDonalds people order and then sit down..but even in this case, your wife or children may sit while you order. If there is a special rule in any restaurant then there may be a sign which asks customers to order before sitting. But I would say this unnecessary and I've never saw such a sign in the UK!

examples "You place your order here " or "You have to order at the counter " or "We take the orders one at a time "

These seats are for paying customers only. By stating the word ‘paying’, this is a simple way to get your point across that you have to buy something in the café to sit down. Following this they would automatically head straight to the counter to make a purchase. “These seats are reserved for paying customers”, has the same meaning. Using ‘reserved’ is more formal and slightly more polite.

I would use the first or second. The third sentence could be a little abrupt.

"Orders are taken at the counter." You need to go to the counter to order what you'd like. "Self service" You need to help yourself at the counter, there are no waiters that help you.