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 );
}