I did twice with same result of non-translation and the file \site\nl-NL.com_kunena.ini is the same as before. It differs from the file en-GB.com_kunena.ini. I have searched the files in NL for 'Mark All Read' and there is no string found.
Download the language package and unzip it. Search your language and unzip also this file. Copy the nl-NL.com_kunena.ini and paste it in your language folder. The original ini file includes 540 lines. Mark all Read is in line 331