User locale doesn't support java8 format

Steps to reproduce

1. Go to https://demopublic.magnolia-cms.com/.magnolia/admincentral#app:jcr-browser-app:browser;/admin/tina/language::  as superuser
2. Switch to users workspace, go to admin/tina/language
3. Change it to a locale-country format (zh-CN)
4. Log out
5. Log in as tina

Expected results

The admincentral is switched to Chinese language. 

Actual results

java.lang.IllegalArgumentException: Invalid locale format: zh-CN
 

Workaround

Use java7 format (zh_CN).

Development notes