I have tried to install from my tmp/ directory the norwegian plug-language package. However I get the following error message
Warning
JInstaller: :Install: Can't find Joomla XML setup file.

Error
Path does not have a valid package.

Unable to find install package

What am I doing wrong?

Hello Gina,
Please try downloading the language pack again.

I did and the same error message appeared again

I managed to install the "Norwegian Bokmål (Norway) language pack for K2" on my site without an issue.

Make sure you get if from -> getk2.org/extend/translations

PS. I took the liberty of installing it on your site as well.

brilliant ;)

You 're welcome Gina.