I like the Palmer-Hughes series of method books, and theyre what I largely used when I teach others. The pace is very good. Theyre not perfect, but I supplement them here and there with other material, substitute my own fingerings in spots, etc. I learned from them myself, so maybe Im a bit biased. Not sure how available this series is across the pond though.
I do have the Gallianos book too, and theres a lot to recommend it. Its written using my preferred left-hand fingering (42), using the pinkie on the major scale (4245353), etc. While it waits an oddly long time to start introducing the chord buttons, when it does, it makes sure to explain the notes that each new chord is made up of, which is great.
Its a pricey book (especially to get shipped here!), but theres a ton of content, including a CD, so still a good value.
It starts you off playing single-note parts in the left hand right away, more like a piano method book would, including bringing in counter-basses very early on. The two hands are kept on par with each other throughout. Its an interesting approach compared to most other method books (which generally relegate the LH to chord patterns for the early and intermediate stages) but Im not quite sure if its my cup of tea.
My main beef with it is probably the songs. Theyre just not that interesting to me in the beginning. Maybe thats a cultural thing. Oh, and it is in French as you pointed out, using fixed-do, which might be a deal breaker to some.
The Seldon books are, I believe, used in Duane Schnurs free online lessons (scroll to the bottom):
http://duaneschnur.com/accordion/lessons.htm Each lesson download contains a chunk of a few pages of the series, so youll have to do a lot of downloading to piece together the whole thing!
Santorella makes a method book series thats currently only up to three volumes. Its not terrible, but it doesnt really wow me either. Maybe better suited to very small children.