To translate them into French, please do the following steps:
Step 1: Go to the file language
Step 2: Copy the folder "En-GB" to "fr-FR"
Step 3: go to fr-FR/ en-GB.com_tz_guestbook.ini and change "en-GB.com_tz_guestbook.ini " into "fr-FR.com_tz_guestbook.ini".
Step 3: translate normally according to the picture mentioned above.
I followed your tutorial and it works perfectly for the translation, but, after the translation, if I click in the in the fields to send a new message, it dosen't delete what is written in the field, you have to delete manually the message in the field.