If you want to force it to be in French despite possible browser settings saying to be in English, you can do that with Javascript.

If you check on the server where LFForms is running, you should be able to find the datepicker language scripts.

• In my case the French one was:
  • C:\Program Files\Laserfiche\Laserfiche Forms\Forms\lib\jquery\i18n\jquery.ui.datepicker-fr.js
  • I don't know if that i18n part changes with different versions of LFForms.
• To access that script from LFForms, the path would be:
  • http://SERVERNAME/forms/lib/jquery/i18n/jquery.ui.datepicker-fr.js

Then use Javascript such as this: 

$(document).ready(function () {
  $.getScript('http://SERVERNAME/forms/lib/jquery/i18n/jquery.ui.datepicker-fr.js', function () {      
  	$('#q1 input').datepicker( $.datepicker.regional[ "fr" ] );
  });
});

Now I see the datepicker showing in French, even though my browser is definitely set to US English.