The original translation file is located here.
To test your translation file in serposcope, you must create a file named conf/messages_en.properties in the serposcope data directory. This file will temporary replace the original english translation for your tests.
Ubuntu/Debian the file should be here :
/var/lib/serposcope/conf/messages_en.properties
Then starts serposcope, and be sure your lang is set to en (top right, Accounts > Preferences > lang).
Each time you edit this file, you must restart serposcope.
To restart serposcope on Ubuntu/Debian :
sudo service serposcope restart