While arguing on French translation with Sébastien. I was surprised, because no accent characters were tipped on the chat dialog. After 5 minutes, I realized that the cause of this bug was the font used for tipping my text. Indeed, the "Bitstream Charter" is not able to display the accent character, whereas the "Serif" can do it perfectly.
I have not tested if other fonts work well and I do not have the time to search/correct this bug for the moment. Maybe someone else is interested in doing some investigations for this?