It could be encoding format of the files, try switching from UTF-8 to ANSI. First set it in lang/fr.php (for example "Windows-1250" does accents really well here).
Then change and save the format of this file as ANSI. Notepad++ does format changing very well. The same with all db files and libraries/Trash.php. Config files too.
oh, you should have said that it displays wrong in the e-mail. I think this problem is solved here? http://opensolution.org/Quick.Cart/forum/?p=readTopic&nr=5406
Neat ! That was it. Not exactly some of all what was written in that post, but actually a missing "change2latin" function when displaying the carrier name (who could create a carrier name with an accent ???) :-)
it has been broken when I swapped to iso-8859-1 so I had to remove it. It was just a sCarriername = change2latin (sCarrierName) ; before the SendEmailWithOrderDetails function call.
As soon as all my testing is fine, I'll send to OpenSolution the whole kit turned to French. I have added in each and every source an audit trail.