Hello Kai,
To change this string into Norwegian language please find this file
Your_joomla_folder\language\en-GB\en-GB.com_payplans.ini
and in this file search for
COM_PP_SELECT_COUNTRY and replace this string with Norwegian text.
Please see attached screenshot.
You can also create a language override file so the changes won't get effected if you update the payplans, Refer to this documentation to know more:
https://stackideas.com/docs/payplans/administrators/customization/creating-language-overrides.
Let me know if you have any other query.