Hi yvesc, 

Download locales file for french from here, https://raw.githubusercontent.com/eternicode/bootstrap-datepicker/master/dist/locales/bootstrap-datepicker.fr.min.js

and then put it inside the js directory.

Now load this file in the footer 

<script src="bootstrap-datepicker.XX.js" charset="UTF-8"></script>

Change the src to correct file path.