Chouby on "[Plugin: Polylang] Translating Custom Post Type & Taxonomy Slugs"

You can try to add the following code in a custom plugin or in the functions.php of your theme. This will work only if you add the language code to all urls.

add_filter('option_category_base', 'pll_category_base');
function pll_category_base($option) {
	if (is_admin())
		pll_register_string('category_base', $option);
	else
		$option = pll__($option);
	return $option;
}

Then you type your original slug as category base in the permalinks panel, and translate it in the strings translation panel.