allow to select the language during installation or later
It seems that Spark tries to use the Windows settings to set the locale settings for a user and the user has no option to set it.
It seems that locale means only i18n files and not date and number formats, it could make sense to adjust also these. Within Germany a 24 hours system is normal, so "2:22 PM" is there "14:22 Uhr"
Current Izpack installer let's user to select the language of installation. Also it is possible to change GUI language through a Actions > Language menu later. Some other issues (time/date formats) should be filed separately. This ticket title speaks only about the language problem.
To change the locale one can set the two Java System Properties "user.language" and "user.region" in Spark.vmoptions. For example:
for US English.
And it seems that Spark queries the Windows settings quite often (probably every time an i18n string is read), so changing the locale within Windows may affect also Spark.