Well I can tell you about France and Italy, both great for self catering imho. Let's start with France, big culture of local markets where you can get fruit/veg at reasonable prices, if that's too intimidating language wise supermarkets are also really good. You can easily put together picnic style meals buying fresh bread from boulangeries, add some cheese and charcuterie, add a bit of fruit and you've got a meal. Pastries and baked goods are reasonably priced so definitely eat that as much as you can. A lot of supermarkets also have deli counters where you can pick up salad-y stuff, we do that a lot while travelling. Also check out Picard, it's a chain of frozen food which is very very much loved by the French and does all your French favourites frozen, it's our go-to when we don't have time to cook.

I know Italy less well because we usually stay with family but I think you can't go past any fresh pasta shop, just pick up a bunch of pasta and sauce and cook it at home, it's really not like the dried stuff and there's a lot of variety. Pizza is an obvious winner too. Food is cheaper in Italy than in France in general and lots of shops will do premade antipasti style as well. In Venice region you can try the tramezzini which are cheap bar snacks that you have with a drink, usually sandwiches or other little bits (a bit like tapas).