For London:In case there's a rainy day (I know, I know, long shot), check out the museums as they're free (they usually do have temporary exhibits you have to pay for though). If you're going to visit the British Museum, make sure to plan ahead and preferably focus on one gallery at a time as you WILL need at least half a day to visit even one.
On a nice day though, I recommend Saint-James Park. It's beautiful, relaxing and it's got plenty of birds (and squirrels expecting food).
if you like markets, Camden and Portobello Road are must-see. Camden is just full of variety with all sorts of cultures and sub-cultures. You can find food, clothing, music, souvenirs and tons of stuff from all over the world. You can even get tattoos and/or piercings if that's your jam. Chinese, Arabic, Hindu, African, Punk, Goth, Hippie, Metalhead, etc... pretty much anything you can think of is represented there.
Portobello Road is more like your typical flea market but you'll find all kinds of interesting and unusual stuff. It's a great place if you want a souvenir that isn't some tacky shit blatantly made for tourists. Hope you're not agoraphobic though because prepare for the place to be packed. Also, it's huuuuuuuuuuuuuge.
For Paris:As soon as you arrive, buy a "Pariscope". I don't know if they have an English-language version but I assume they do as these are booklets that give you info on pretty much every cultural event going on in the city. Theatre plays, museum exhibits, etc... so if you want to find out what to check while you're there and you may not have another occasion to see, it's a neat help.
The Louvres is of course a classic but like the British Museum, I advise focusing on one area at a time. Also, get there early in the day if you want to avoid long lines.
Check out a Greek restaurant called "Le Souvlaki Athenien" which is located at the 25, Rue de la Huchette. I know when foreigners visit Paris and France in general, they want to try French food but trust me, it's a great one. Oh and in the same street, there is an ice cream shop called Amorino that makes insanely big AND tasty ice cream so that's something to look forward to for dessert.
I'd also advise visiting the Centre Pompidou. The exhibits there are kind of hit or miss but usually good. Plus they have great stores there and if you still don't enjoy it, it's always surrounded by all sorts of street performers and artists (no, not mimes). And right next door to it is the "Crêperie Beaubourg". It's a small restaurant that makes simple but delicious dishes. If you're looking for traditional Parisian stuff, this is pretty much exactly what you want.
Paris also has a lot of great flea markets (like Clignancourt and Saint-Ouen) which, come to think of it, are reminiscent of Camden (although not quite as impressive, in my opinion). Again, you should be able to find plenty of interesting stuff rather cheap.
Ironically, I can't really answer your two questions as I have never been to Disneyland Paris and being a native French speaker and all, I don't really speak English when I visit there.
the language barrier shouldn't be much of an issue though; you should fairly easily find people who speak at least a little bit of English and Parisian people are more helpful than their reputation would lead you to believe. Also, stores and public locations almost certainly have at least one people speaking English considering the amount of tourists they have to deal with. Could be nice to learn a few words though, even if it's just "Bonjour! Excusez-moi, parlez-vous Anglais?". People appreciate the effort.
