Translation not applied

The language of the site is for Faroe Islands and is not supported natively by WordPress.

The back-end is set to "Dansk" (Danish) and Danish translation files were updated - translated strings were changed to the Faroe Islands language. However, site is not picking up these translation.

To replicate: upload (attached to the next post) files to /wp-content/languages/plugins folder, enable MarketPress and set WP to use "Dansk" language. None of these translations take effect, except for the message that IDEAL and some other payment gateways had been disabled.