Full text of medieval charters of communes in Northern France?

Firstly, I'm going to assume from your post that you don't speak/read French. How about Latin?

If that's a no too, then you're in for a tough time finding anything helpful. There is no central repository for French communal/urban charters and many of them havn't even been published, let alone been put online. Those that have been published (either in print or online) will usually be kept in the original Latin or, much less likely, translated into French. I can't say I've ever come across any that have been translated into English.

There are some options open to you if you want to try get your hands on some texts, but to get a lot of them it'll probably take access to some sort of inter-library loan service. Firstly, you can start with Stein's Bibliographie Generale des Cartulaires, which is online here. Stein is a listing of all the cartularies (charter collections) preserved in France and is invaluable for getting a sense of what's out there. It's organised alphabetically by placename, and thereafter by institution, so you can scroll down to the town you want (Arras, Rouen, Paris) and then find the section for the town itself ('Ville'), nestled in amongst all the ecclesiastic institutions. You can then do a google search for the document or cartulary you've identified, which may either be online or gathering dust in an archive.

Which brings me to the second route you can go down, which will be particularly useful for finding original charters (i.e. those not preserved in cartularies): go visit the websites of the Archives Départementales and the Bibliothéque Municipale of the towns you're interested in. This can be an exercise in frustration, and many will not have an English-language page or even any helpful information, but if there's going to be anything published online, either the full charter texts or information about them, it's likely to be here.

Those would be my starting points anyway. Good luck with whatever you're researching and feel free to shoot any follow-up questions my way: I've spent the last few weeks trying to identify obscure French charter sources, so I should be able to give pointers if nothing else!