Use WordPress default text domain for translating labels

If you don’t provide any textdomain, WordPress translated strings are used. If you use, for example, __( 'Add New Tag' );, the translation from WordPress core will be used because 'Add New Tag' exists in WordPress.

Your problem is that your are defining labels that don’t exist in WordPress core, so they are not included in any WordPress translation files. I mean, strings like 'All Items' or 'Popular Items' don’t exist in WordPress and they are not translated anywhere.

I think my explanation is a little tricky, I hope you get what I mean.