Language file outside plugin not working

Hi

You write this in your cgangelog:
4.0.2
Added a new option so you can store your custom language file for this plugin in a folder outside the plugin’s directory.

My localized language files are placed here:
http://www.xxxxxx.dk/wp-content/languages/plugins

However the front end txt is still in English.

My issue is that:
My WP dashboard must run in English, but the front end of the the shopping cart plugin must be shown i Danish.

Could you advice me on how I might make this work?

https://wordpress.org/plugins/wordpress-simple-paypal-shopping-cart/