Translated my theme (translation not showing up)

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' );
?>