Alexandre,
Sorry you're running into that issue. My guess is that the string isn't defined in your language file, for some reason. JFBConnect should fall back to the English translation if that string doesn't exist, but it's possible we're doing something wrong in your case.
That string should be defined in /language/xx_YY/xx_YY.com_jfbconnect.ini Can you check that file, where xx_YY is your language for your site, and search for COM_JFBCONNECT_USER_MAP_SUCCESS. It should be in the file and *NOT* have a ; in front of it. If it has a ;, remove that and then update the right-hand side of the string to whatever text you want to display.
Let me know how that goes, and if you still have issues, just let me know!
Thanks,
Alex