Secondly, big thank for your help. YES ! It works, adding 'entity_encoding : "raw",' in the header.php file resolves my problem, I have now 'é' instead 'é' in my rich text fields and searching words with such caracthers becomes very more easy. I have now to recall all my records already in my DB (one by one I suppose) and simply register them without changing anything, So they will be ok.
And also, I found the "browser_spellcheck : true," parameter that permits me to limit greatly my spelling errors

I have now to review all the parameter to see if some other ones could be useful for me.