According to the Locale Java 7 javadoc:
Java has used ja_JP_JP to represent Japanese as used in Japan together with the Japanese Imperial calendar. This is now representable using a Unicode locale extension, by specifying the Unicode locale key ca (for "calendar") and type japanese. When the Locale constructor is called with the arguments "ja", "JP", "JP", the extension "u-ca-japanese" is automatically added.
This results in a ja_JP_JP_#u-ca-japanese Locale which is currently parsed as invalid. Note that there is a similar issue for Thai.