Dark side dialog editing- need help

[bobotjo]

I want to edit some of the dialog in loc_ses_eng.kfs to personalize the game, but I cannot get it to work.
I tried editing the .kfs in the Darkside folder. I tried puting edited versions of loc_ses_eng.kfs or loose .lng's in darkside\mods.
Nothing seems to work. The moment I touch a any eng_chatxxx.lng file that specific dialog is no longer recognized by the game.
The dialog still 'works', but the text strings are replaced by white error messages.
What am I missing?
I have the Premium edition (steam version) of the game.