How should you coordinate with traffic at non-towered airports?
The problem with uncontrolled airports is that sometimes the procedures and "etiquette" can be informal.  The pilots based at the airport have established their own informal procedures which are not published anywhere or are part of any rule book.  In cases like this I would consult the FBO for the local procedures (which you did do) but that doesn't mean that all the pilots at the airport abide by them.  In your situation this is what I would have done:


I would have announced on the CTAF that I cannot speak French and would request that all pilots announce their intentions in English.  By international convention pilots are required to communicate to ATC via English unless there is an emergency.  However CTAF communications tend to be more informal and the pilots at the local airport may prefer to speak in the language they are the most comfortable.  Still the intent of CTAF is to communicate with everyone at the airport to announce their intentions.  It would have been entirely proper to request the pilots to speak in English.  
I would have requested the pilots performing the touch and goes to give you a window in which to back taxi and takeoff.  I find it rather rude for pilots to monopolize the pattern and runway performing touch and goes.  If they see that there is a line of planes wanting to takeoff they should leave the pattern for a while and let folks takeoff before resuming their touch and goes.  Asking politely on the radio for these pilots to do this for you is reasonable.  
If they were giving you grief for not using the grass to taxi to the runway they had no right to do that.  Taxing on grass can be dangerous especially in tricycle gear aircraft where the potential for a prop hit with the ground is a real possibility.  In my case I am bound by the club rules not to taxi on any surface that is not paved.  Back taxing would be the appropriate thing to do.