Allow custom display names for locales in LocaleProviderService

In CMS-9701 we removed the capability of customizing the locale display name translation because this is functionality that is built-in in java. However, the hippo connect, and possibly other projects, use non-built-in 'locales', in the case of hippo connect to have different versions of the documentation: 7_9, 10, etc. For this use-case we need a display language override mechanism.