Wordpress - How to intercept already localized scripts

wp_localize_script() calls the method localize() on the global variable $wp_scripts. We can set this variable to an instance of a child class of WP_Scripts:

class Filterable_Scripts extends WP_Scripts
{
    function localize( $handle, $object_name, $l10n )
    {
        $l10n = apply_filters( 'script_l10n', $l10n, $handle, $object_name );
        return parent::localize($handle, $object_name, $l10n);
    }
}

add_action( 'wp_loaded', function() {
    $GLOBALS['wp_scripts'] = new Filterable_Scripts;
});

The theme customizer doesn’t use that, it creates a separate instance of WP_Scripts (see wp-admin/customize.php). It might be possible to replace that too:

add_action( 'customize_controls_init', function() {
    $GLOBALS['wp_scripts'] = new Filterable_Scripts;
    $GLOBALS['wp_scripts']->registered = $GLOBALS['registered'];
});

None of this has been tested, just an idea.

@toscho great implementation. Tested and true. Here is a slightly modified version, which also passes the $handle and $object_name so you can filter only when needed.

class Filterable_Scripts extends WP_Scripts
{
    function localize( $handle, $object_name, $l10n )
    {
        $l10n = apply_filters( 'script_l10n', $l10n, $handle, $object_name );
        return parent::localize($handle, $object_name, $l10n);
    }
}

add_action( 'init', function() {
    $GLOBALS['wp_scripts'] = new Filterable_Scripts;
});

add_filter('script_l10n', 'se108362_example_filter', 10 , 3);

// Example
function se108362_example_filter($l10n, $handle, $object_name ) {
    if('js-handle' == $handle && 'jsVariable' == $object_name) {
       return 'Something Else';
    }
    return $l10n;
}