Currently the following additional languages are included: Danish, German, Italian, Dutch.

To change ShiftExec to one of those languages, please edit the following file:

happ/hclib/hc_lib.php

change the line 130 so it reads as:
$locale = "it_IT";
(to change it to Italian for example)

If you language is not already there, your would need to translate the language file. ShiftExec uses the same mechanism for localization as WordPress does. It's required to make the .mo/.po files, there's a nice software called Poedit that makes it a breeze.

In ShiftExec folder there's a subfolder named "languages", you will need to start the Poedit software, choose the "Create new translation" option and select the "shiftcontroller.pot" file. It will prompt you to choose the language you are translating to, then as you complete the translation, it will create 2 new files: shiftcontroller-it_IT.mo and shiftcontroller-it_IT.po. Once they are in the "languages" directory, you can change the hc_lib.php file as described above.