Thank you Frenzie for your detailed answer.
Of course for SSH over wi-fi! I saw the usbnet package in the jailbreak thread, so I forgot about it, but it is obvious!
Yes, I tried to change language to fr and it works.
Do you know if "koreader/frontend/ui/language.lua" is the only place where it is specified? If yes, I will only modify this file and rename the "fr" folder...
I will look into the font issue, thanks.
[Edit] Ok, it explain the problem without resolving it. Not a serious problem.