You have incomplete code. You register your Theme’s textdomain, but don’t actually tell WordPress to load your translation files.
To this:
load_theme_textdomain('INTERluminaires', get_template_directory() . '/languages');
Add this:
$locale = get_locale();
$locale_file = get_template_directory() . "/languages/$locale.php";
if ( is_readable( $locale_file ) ) {
require_once( $locale_file );
}
Or, altogether (and wrapped properly in a callback):
<?php
function wpse49326_translate_theme() {
// Load Theme textdomain
load_theme_textdomain('INTERluminaires', get_template_directory() . '/languages');
// Include Theme text translation file
$locale = get_locale();
$locale_file = get_template_directory() . "/languages/$locale.php";
if ( is_readable( $locale_file ) ) {
require_once( $locale_file );
}
}
add_action( 'after_setup_theme', 'wpse49326_translate_theme' );
?>