Hey Marco,
I am really sorry for the delay of this reply as it is a weekend for us here. Okay it looks like these strings weren't translated in the theme file
Are you on 2.0.5 already? In 2.0.5, we have fixed the following files to use a new translation string:
/components/com_komento/themes/wireframe/comment/form/cancelbutton.php
/components/com_komento/themes/kuro/comment/form/cancelbutton.php
The language string used is:
COM_KOMENTO_FORM_CANCEL