Can't add external rewrites

First off, don’t ever do this:

add_action('init', 'flush_rewrite_rules');

Every time WordPress loads, you are going to flush the rewrite rules. You only need to do this once to make sure your rewrite shows up.

Second, add_rewrite_tag only takes two arguments, the tag and the regex. If you want to specify a query variable directly, you’ll need to do something like this:

<?php
global $wp_query;
$wp_query->add_rewrite_tag('%gallery%','([^/]+)', 'gallery=');
$wp_query->add_rewrite_tag('%album%','([^/]+)', 'album=');

In your case, it doesn’t really matter: you’re using the same query variable as the tag.

Finally, there were some errors in your regex. You need to start with a carrot, and I wouldn’t use (.*) for second part. You also need to specify the third argument of add_rewrite_rule which tells WP where to place the rewrite in prominence. You want top to tell WordPress that this rule should come before the built in rules.

This works:

<?php
add_action( 'init', 'wpse41778_add_rewrite' );
function wpse41778_add_rewrite()
{
    add_rewrite_tag('%gallery%','([^/]+)');
    add_rewrite_tag('%album%','([^/]+)');
    add_rewrite_rule(
        '^galleries/([^/]+)/?$', 
        'index.php?pagename=galleries&album=1&gallery=$matches[1]',
        'top'
    );
}

Provided you flush the rewrite rules. If you drop the rules into a plugin you can use register_activation_hook to flush them.

<?php
register_activation_hook( __FILE__, 'wpse41778_flush_rewrite' );
function wpse41778_flush_rewrite()
{
    wpse41778_add_rewrite();
    flush_rewrite_rules();
}

To get the value of the gallery or album query variables you can do this:

$gallery = get_query_var( 'gallery' );
$album = get_query_var( 'album' );

Based on your rewrite, your usage is probably going to be something like this:

<?php
if( get_query_var( 'album' ) )
{
    $gallery = get_query_var( 'gallery' );
    // do stuff with $gallery
}

Here is the rewrite part of that as a plugin.

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