The Derby documentation refers to three locales.
This is the locale of your machine, which is automatically detected by your JVM. For Derby and Derby tools, the Java system locale determines the default locale.
This is the locale associated with your database when it is created. By default, this is the same as the Java system locale. The database locale determines the language of database errors.
This locale is associated with your session, when using Derby tools such as ij or dblook. This locale determines the language of messages, as well as the localized display format for numbers, dates, times, and timestamps. You can use the derby.ui.locale property to specify the session locale that should be used.