Not all strings are translation ready

I have just spent some time translating the PO file to nl_BE but I notice that a few strings still don’t translate. Not even when I switch to one of the already available locales.
For example the “Add to quote” button on the WooCommerce product. When I look at the PHP code in add-to-quote-button.php I see a simple $label instead of something like <?php _e( ‘Add to quote’, ‘yith-woocommerce-request-a-quote’ ) ?>. FYI when I replace $label with that echo, I do see my translation so my MO-file is good.

It looks like the plugin is not 100% translation ready.