Posté en anglais ou en français ne change rien au problème !
Non, mais c'est la règle de base du Forum : poster en anglais pour que les réponses puissent profiter au plus grand nombre.
Je répondrai donc en anglais.

___________________________________________________
The problem you get with accentuated caracters do not seem to be linked to database.
The problem appears on captions, which are stored on files (exactly on /tool/i18n/nls/xx/lang.js files)
So first check for /tool/i18n/nls/fr/lang.js. It must be ANSI AS UTF-8. And accentuated caracters must appear correctly.
Is it the case ?
What is stange is that captions on menu items are correct (for instance "Paramètres globaux") and the same on Screen Name (beside save button) is incorrect : but both come from same lang.js file, same line

Only difference is that first one is just a HTML page with standard request, and second one is loaded using Ajax.
Do you have any specific configuration about locale, or UNICODE proxy or something like that ?
Could you try to check on Demo version : do you reproduce issue (then it is linked to your browser configuration) or not (then it is linked to your server - or network - configuration).