"Additional info" don't get translated.
Although I did change the language files
en-GB.com_k2
en-GB.mod_k2_content
And in my language files

How can I change this?

Hi. First of all in order to translate you do not change the original files, you create corresponding language files for the desired language. Secondly you can try to enable "Debug System" and "Debug Language" options under Joomla! configuration which will give you some helpful information about variables that are not translated.