I had an awkward experience buying some produce in a Paris grocery store when I'd totally forgotten that I needed to weigh and get a barcode in the produce section instead of having the cashier weigh items like here in the US.
I've also read that in Dutch grocery stores, you bag your groceries yourself and it's wise to put items on the conveyor belt in the order they'll be packed up because other customers can get sort of fussy if the person ahead of them is slow. It sounds like some chains like Carrefour, in The Netherlands at least, give customers the option of a handheld scanner so you shop and scan and bag all at once, then hand off the scanner to a cashier to pay. It seems like you need to have some sort of store bonus card or frequent shopper card or similar to be eligible to use the scanner though.
I think just about everywhere, customers are using their own reusable bags.
I have a bit of social anxiety and foods are my main souvenirs so what else is useful to know about grocery stores ahead of time?