This feature enhances java.util.Locale and its related API to implement an additional Unicode extension of the BCP 47 language tag. Support for the BCP 47 language was added to JDK 7. However, in JDK 7, support for the Unicode locale extension was limited to calendar and numbers. This feature allowed for the addition of an extension to a locale. JDK 9 added support for ca and nu tags, from BCP 47.
JDK 10 added support for the following extensions:
- cu (currency type)
- fw (first day of the week)
- rg (region over)
When you specify a locale extension, such as a number or currency type, it is not guaranteed that the underlying platform supports the requested extension. Unicode tag extensions are a language feature, and they can be used directly by the developers.