language setting

I am using the theme Responsive Pro, and want to change the default language setting to work in French. What do I enter? I have tried “Resposive Pro”, but still get English terms (like “Reply”) in the Comment page. NB the main contact form is fine, but follow-up emails are all in English too.

My site is currently test.hanvecforum.fr
Click on the “Exprimez-vous” box on the home page to see the comments.

https://ww.wp.xz.cn/plugins/comment-guestbook/