Conditionally load YOURLS lang

<?php
/*
Plugin Name: FR or EN ?
Plugin URI: http://yourls.org/
Description: Load FR or EN depending on something
Version: 0.1
Author: Ozh
Author URI: http://ozh.org/
*/

// Hook into 'get_locale' filter
yourls_add_filter( 'get_locale', 'fr_or_en_select' );

// Return 'fr_FR' or 'en_US' depending on something
function fr_or_en_select() {
    // test something here
    if( ...?.... ) {
        $locale = 'fr_FR';
    } else {
        $locale = 'en_US';
    }

    return $locale;
}