Add new language programmatically

Hello,

A language can be added this way:

$options = get_option( 'polylang' );
$model = new PLL_Admin_Model( $options );
$model->add_language( $args );

For the documentation of what to enter in the array $args, see the description of the method add_language: https://github.com/polylang/polylang/blob/master/admin/admin-model.php#L10-L30

This method is for example used to create languages in our phpunit tests: https://github.com/polylang/polylang/blob/2.6.8/tests/phpunit/includes/testcase.php#L33-L43