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');