I am trying to get the word 'on' translated into a norwegian word in the recent comments widget in the sidebar on my page.
I've looked in the bfa_recent_comments.php file, but I don't know what to change.

Your using the 'Recent Comments' widget right, not the 'BFA Recent Comments' widget
it's wordpress - you need to edit wordpress/includes/default_widgets.php line 651 where you will see

...sprintf(_x('%1$s on %2$s', 'widgets'), ....

if you are using the 'BFA Recent Comments' widget, what settings do you have set for it?

Editing helped instantly, thanks a lot!

I sent a small donation from a poor - but now very happy - norwegian student!