For some reason the backend shows in multiple languages sometimes. Also when visiting the site and switching language, that language will remain when going back to WP-admin. Obviously as a Dutch person I don't feel like working in a French, Italian or German WP Admin area.
If I understood correctly, you want the WordPress Admin in Dutch, despite the default language or any other language selected on the frontend, is this correct?
If yes, please note that since WordPress version 4.7, the option to set the administration language is provided natively by WordPress.
However, if this isn't the case, can you please explain the issue in more detail for better understanding? I'll try my best to further assist you on the matter.