To change the used language, rename the languagefile to default.lng
or start acarsdclient with the commandline:

FOR EXAMPLE WITH GERMAN LANGUAGE
acarsdclient -lang german.txt

To compile a new language file, open the example files (english.txt or
german.txt) within your texteditor and edit the strings.
Please note that between the indicator (numeric value) and the string
must be one TAB
After you have changed all strings, open a DOS Window (command or cmd)
and change into your acarsdclient folder.
Here you can start the language compiler:

FOR EXAMPLE WITH ITALIAN LANGUAGE:
langcomp italian.txt italian.lng

Done. Now start acarsdclient with the -lang option our rename your new
language file to default.lng

