ソースは上にかけますか？別にしますか？って英語でなんて言うの？

By using this phrase you are asking the customer if they would like the sauce mixed into their food or "on the side". "On the side" is very common when asking a customer if they still want the sauce with their meal but they can mix in their own preferred amount.

You can use any of these examples to ask someone if they want the sauce on the food or on the side. You can even ask, "Would you like your sauce in a container?" They will just say, "yes." or "No, on top is fine."

In the UK, sauce is almost always served in a pot if you have a takeaway. There are a number of questions arising if someone else is to put the sauce on your meal. How much sauce do they put on? Do they put all the sauce in one area, or do they squirt the sauce all over? However, the example given is adequate if you need to ask that question.

"Would you like the sauce on the side?": You can use this to ask if the customer would like any accompaniment of their order on the side or in the meal.

All of these questions will get the needed answer from the customer when you are talking to them. When I take a takeaway (or "to go" in American English) I will often make helpful suggestions such as: I think it might be good if I put the sauce on the side. I'll put the sauce on the side for you so it doesn't get soggy. I put the sauce in a separate container for you. Customers usually appreciate this.

"Would you like the sauce in the bag or on your food?" This is another way of asking where they would like to have their sauce. "How would you like your sauce?" This is a polite way of asking how they would like their sauce, and gives them a chance to explain what how they would like it.