In the output folder there is now a #####_eng.pot file with all strings. Level names start at string 202.

If you want other languages, you can copy this file and name give it the language name of your choosing. For example, name it text_ger.po if your language of choice is German.
Then edit this .po file with a text editor or POEdit and put in all the translations in the msgstr strings. Examples can be found here: https://github.com/dkfans/keeperfx/tree/master/lang/campgns/ancntkpr
Keep them in the output folder.

When you are done, double click 4_CreateTranslationFiles

Now you are finished. Put the .dat files and the updated .txt files back in your campaign levels folder and update your campaign config file to use the .dat files. I strongly recommend keeping the .po and .pot files and bundling them along with the campaign in case future modifications need to be made.