You can add your own language into the program. The only thing you have to do is to place the file properly and respect the format of the file.
Format of the language file looks like this (the template is located in Languages/template/template.properties file):
BUNDLE_LANGUAGE_NAME=Deutsch
WINDOW_TITLE=PSimulator
FILE=File
EXIT=Exit
NEW_PROJECT=New Project...
CLOSE=Close
OPEN=Open...
OPEN_RECENT_FILE=Open recent file
EMPTY=Empty
SAVE=Save
SAVE_AS=Save As...
LANGUAGE=Language
PREFERENCES=Preferences
...
Each row contains a key (FILE) and a value (File). You have to translate all values. The name that will be displayed in the app in language selection is on the first row (BUNDLE_LANGUAGE_NAME).
When you translate all values and when you do not delete any key and value, than your file will be recognized by the app as a valid one. Valid languages will be offered in language options.
Your language file has to be located in Languages directory: PSImulator2/Languages/ (located in PSImulator2 directory next to the .jar files).