Wordpress - Theme localization of "slugs" (custom post types, taxonomies)

I wouldn't try to localize your slugs. Instead, why not give your users the option to change them by adding another field to the permalink settings page?

Hook into load-options-permalink.php and set up some things to catch the $_POST data to save your slug. Also add a settings field to the page.

<?php
add_action( 'load-options-permalink.php', 'wpse30021_load_permalinks' );
function wpse30021_load_permalinks()
{
    if( isset( $_POST['wpse30021_cpt_base'] ) )
    {
        update_option( 'wpse30021_cpt_base', sanitize_title_with_dashes( $_POST['wpse30021_cpt_base'] ) );
    }

    // Add a settings field to the permalink page
    add_settings_field( 'wpse30021_cpt_base', __( 'CPT Base' ), 'wpse30021_field_callback', 'permalink', 'optional' );
}

Then the call back function for the settings field:

<?php
function wpse30021_field_callback()
{
    $value = get_option( 'wpse30021_cpt_base' );    
    echo '<input type="text" value="' . esc_attr( $value ) . '" name="wpse30021_cpt_base" id="wpse30021_cpt_base" class="regular-text" />';
}

Then when you register your post type, grab the slug with get_option. If it's not there, use your default.

<?php
add_action( 'init', 'wpse30021_register_post_type' );
function wpse30021_register_post_type()
{
    $slug = get_option( 'wpse30021_cpt_base' );
    if( ! $slug ) $slug = 'your-default-slug';

    // register your post type, reference $slug for the rewrite
    $args['rewrite'] = array( 'slug' => $slug );

    // Obviously you probably need more $args than one....
    register_post_type( 'wpse30021_pt', $args );
}

Here's the settings field portion as a plugin https://gist.github.com/1275867

EDIT: Another Option

You could also change the slug based on what's defined in the WPLANG constant.

Just write a quick function that holds data...

<?php
function wpse30021_get_slug()
{
    // return a default slug
    if( ! defined( 'WPLANG' ) || ! WPLANG || 'en_US' == WPLANG ) return 'press';

    // array of slug data
    $slugs = array( 
        'fr_FR' => 'presse',
        'es_ES' => 'prensa'
        // etc.
    );

    return $slugs[WPLANG];
}

Then get the slug where you register your custom post type.

<?php
add_action( 'init', 'wpse30021_register_post_type' );
function wpse30021_register_post_type()
{
    $slug = wpse30021_get_slug();

    // register your post type, reference $slug for the rewrite
    $args['rewrite'] = array( 'slug' => $slug );

    // Obviously you probably need more $args than one....
    register_post_type( 'wpse30021_pt', $args );
}

The best option, IMO, would be to both give the user an option and provide solid defaults:

<?php
add_action( 'init', 'wpse30021_register_post_type' );
function wpse30021_register_post_type()
{
    $slug = get_option( 'wpse30021_cpt_base' );
    // They didn't set up an option, get the default
    if( ! $slug ) $slug = wpse30021_get_slug();

    // register your post type, reference $slug for the rewrite
    $args['rewrite'] = array( 'slug' => $slug );

    // Obviously you probably need more $args than one....
    register_post_type( 'wpse30021_pt', $args );
}

If that doest not work Why not you just simple do:

$post_slug=  __('product', 'mytextdomain');
'rewrite' => array( 'slug' => $post_slug );

I am doing exactly that in a theme we are developing. It is available in 5 distinct languages, and each language has a translated set of categories. The first component of the URL in the theme is parsed to determine which language is used, in country-language format:

/uk-en
/fr-fr
/it-it

And then translated categories are parsed as further components of the URL.

The URL is parsed in the parse_request phase:

function my_parse_request( $wp ) {
    $path = parse_url( $_SERVER['REQUEST_URI'], PHP_URL_PATH );

    $components = preg_split('|/|', $path, null, PREG_SPLIT_NO_EMPTY );

    // Determine language from $components[0]
    $language = array_shift( $components );
    ...

    // Load translations file...
    $mofile = get_stylesheet_directory()."/$language.mo";

    load_textdomain( 'mydomain', $mofile );

    ...

    // Determine category from $components[0]
    if( __( 'some-category', 'mydomain' ) == $components[0] )
      $wp->query_vars['category'] = 'some-category';

    ...
}
add_action( 'parse_request', 'my_parse_request' );

This example is devoid of requisite checks, but is meant only as an example.

There are drawbacks to this approach, of course, but it allows natural URLs in all languages. The main drawbacks I see are:

1) It doesn't make use of the permalink mechanism. This could likely be extended so that the proper permalink rules for all languages are generated and parse_request won't be necessary, but to do it for all of the languages would involve loading one MO file after another in a loop, and I don't know how well supported that is.

2) If a translator changes a slug, then the links get invalidated.