The constant _LANGUE_PAR_DEFAUT sets the default language. The default value is French (code "fr"). For some non-French-speaking sites, it may be interesting to set another value.
It is possible to customize this value in your file
config/mes_options.php (See the article dedicated to it).
Example :
// the default language is Dutch
define('_LANGUE_PAR_DEFAUT', 'nl');