I'm in Belgium, but it's pretty much the same here as in France/Paris, so I'll have a go at answering your questions....
Are restaurants un-childfriendly? No, not really. But, don't expect massive child menus or tons of facilities. People don't fuss the same as in the UK. There might be a small kids menu for example, or you pick a dish from the "side dishes" or, as we now do, get an adult portion and split it between you (we split 1 adult portion between our 3 young kids).
What do people do about feeding and changing babies? There might be a drop down baby changing table in the ladies' toilet or the disabled toilet, depending on the restaurant. Otherwise, I have a fold up changing mat which I have often used on the floor of the toilet. For highchairs, many places will have them, or will have the small booster-type seats to put on chairs. If you're concerned, you could always buy a travel booster seat (we have an inflatable one we used a bit for DS1).
Can you get the same formula over there? Depends on the brand. Some very popular brands here (Nestlé for example) are not sold in the UK, and vice-versa (SMA isn't sold here for example). Many brands are sold under different names in different countries (for example, Aptamil is sold as Nutrilon here/France). Even between France and Belgium the names often vary, so you might want to bring your own formula. The rules for making formula also differ massively from country-to-country (here, for example, the advice is to prepare formula with lightly mineralised water).
Can you get pouches for convenience? Pouches of food are becoming more popular - you can get some Ella's kitchen stuff here and HIPP pouches are often available. However, mostly the supermarkets sell the jars rather than pouches. For milk, pre-made up formula is NOT readily available for younger children/babies. A friend of mine used to buy it in the pharmacy, but you'll only see the cartons of formula for children over 1 (lait de croissance) in the supermarkets.
Are nappies really expensive? Not sure what you class as "really expensive", but they tend to be more expensive than in the UK (perhaps less so now because of the weak pound). For comparison, I would pay €22,95 for a pack of 64 size 4 Pampers baby dry, and they cost about the same in France.
Funnily enough, on a recent visit back to the UK I found UK restaurants, cafés etc to be much less baby-friendly than here on the whole. I needed to heat up a tupperware pot of baby food for DD, and couldn't find any places to do it. Everywhere, except one cafeteria-type restaurant in a shopping centre, told me they weren't allowed to do it because of health and safety concerns (in case they made the food too hot and then I sued them for hurting DD basically). They were all happy to give me a pot of scalding water to put the pot in though - far more risky and bloomin' useless when you're heating mushed up home-made food, not liquid). Over here, most cafés and restaurants will happily heat up bottles and baby food that you bring with you. So, although they might not have a baby changing table, it's actually a lot easier to take my family out for lunch etc here as I can actually feed all my kids :-)