I assume that you are a French girl. You have two things to do. The second is to set a proper locale setting. If your locale setting is proper, you can see French messages. If you can't, you might have a wrong locale setting.
Some environment variables affect.
If you are not familiar with locale, I suggest you to set LANG environment variable as follows. Message translation is not so difficult. If you think gtkdiff is useful, I would like you to translate the messages, and send it to me. I assume that you are an Italian girl. Moreover, I assume that you are familiar with Emacs.