I assume that same locale diagnostic output indicated that the Joomla language and the JCalPro language was french. If that is so ...
Look in "languages/french/index.php", around line 40, you should see the following:
,'locale' => array('fr','french') // Standard locale alternatives for a specific language. For reference, go to: http://www.php.net/manual/en/function.setlocale.php
The strings in the arry ('fr' and 'french') are the locale strings that JCalPro is trying to get your web server to match. Your server is responding that these are not available, so you need to find something else.
If you can identify the actual locale options available on your server, that is best. If not, try this:
,'locale' => array('fr_FR', 'fr_FR.UTF8','fr','french') // Standard locale alternatives for a specific language. For reference, go to: http://www.php.net/manual/en/function.setlocale.php
Other choices for locale strings to try would include the following, but they appear to be other country variants:
fr_BE
fr_CA
fr_CH
fr_BE.ISO8859-15
fr_CA.ISO8859-15
fr_CH.ISO8859-15
fr_FR.ISO8859-15