How change or configure default localize language for a DataPicker “com.codename1.ui.Calendar”
To localize strings for month names use the values "Calendar.Month" using the 3 first characters of the month name in the resource localization e.g. "Calendar.Jan", "Calendar.Feb" etc... To localize strings for day names use the values "Calendar.Day" in the resource localization e.g. "Calendar.Sunday", "Calendar.Monday" etc...
From https://www.codenameone.com/javadoc/com/codename1/ui/Calendar.html
When we say localize we mean a resource bundle: https://www.codenameone.com/javadoc/com/codename1/ui/plaf/UIManager.html#setBundle-java.util.Map-