Loco Translate: Custom Post and Custom Taxonomy Labels not translated in wp-admin menu

Question

I am currently translating all of my plugins with loco translate. This is working fine on frontend and in all metaboxes in the backend, but not in the wp-admin menu:

The red circled word should be in german but isn't.

My Custom Post Type:

<?php

function add_ons() {
    register_post_type('add_ons',
        array(
            'labels'      => array(
                'name'          => __( 'Add-Ons', 'add-ons' ),
                'singular_name' => __( 'Add-On', 'add-ons' ),
            ),
            'taxonomies'  => array( 'add_ons_category' ),
            'public'      => true,
            'has_archive' => true,
            'show_ui' => true,
            'rewrite' => array(
                'slug' => 'add-ons'
            ),
            'supports' => array(
                'title',
                'editor',
                'post-thumbnail',
                'custom-fields',
                'revisions'
            )
        )
    );
}
add_action('init', 'add_ons');

function add_on_category() {
    register_taxonomy( 'add_ons_category', 'add_ons', array(
        'label'        => __( 'Add-On Category', 'add-ons' ),
        'rewrite'      => array( 'slug' => 'add-on-category' ),
        'show_admin_column' => true,
        'hierarchical' => true,
    ) );
}
add_action( 'init', 'add_on_category', 0 );

The text domain is correct and I have translated the strings in Loco Translate, they are just not showing in the menu. But they are shown anywhere else where they are used.

Any ideas?

Answer 1

For anyone having the same trouble, I am now using a workaround from the accepted answer on the following question: https://wordpress.stackexchange.com/a/30723/181214

I am using the solution beyond the "EDIT: Another Option" which is working fine for slugs, names, singular names etc.

function get_labels() {
    // return a default slug
    if(!defined('WPLANG') || !WPLANG || 'de_DE' == WPLANG) return array('slug' => 'produkt', 'name' => 'Produkte', 'singular_name' => 'Produkt');

    $slugs = array(
        'en_US' => array('slug' => 'product', 'name' => 'Products', 'singular_name' => 'Product'),
        'en_UK' => array('slug' => 'product', 'name' => 'Products', 'singular_name' => 'Product'),
        'de_DE' => array('slug' => 'produkt', 'name' => 'Produkte', 'singular_name' => 'Produkt'),
        'de_CH' => array('slug' => 'produkt', 'name' => 'Produkte', 'singular_name' => 'Produkt'),
        'fr_FR' => array('slug' => 'produit', 'name' => 'Produits', 'singular_name' => 'Produit'),
        'es_ES' => array('slug' => 'producto', 'name' => 'Productos', 'singular_name' => 'Producto')
    );

    return $slugs[WPLANG];
}

function product() {
    $labels = get_labels();

    register_post_type('product',
        array(
            'labels'      => array(
                'name'          => $labels['name'],
                'singular_name' => $labels['singular_name'],
            ),
            'public'      => true,
            'has_archive' => true,
            'show_ui' => true,
            'rewrite' => array(
                'slug' => $labels['slug']
            ),
            'show_in_menu' => true,
            'supports' => array(
                'title',
                'post-thumbnail',
            )
        )
    );
}
add_action('init', 'product');