Thank you Senter, but before flashing the phone, I would like to find (or take from the phone if possible) the necessary files to be able to put it back to its original configuration.

So I think I need to find out the MAIN, FS and CUSTOM files corresponding to a K770, R8BA024, ORANGE, FRANCE. Am I wrong?

Afterthat, when I will be sure to be able to go back, then I will try to flash it with the files you propose.