Rewrite the default language setter to remove the need to call flushCache

The default language updater script uses protected API and it calls flushCache() in order to cleanup the mess it may have set.

In recent versions of XWiki, flushCache triggers a reinitialisation of the wikis on next access after a flushCache, and this can be bothering.

Also, in recent versions (XWIKI-15159), a public API to set the default language of a document was added, and we should update this script to use that, there is no need to continue using protected API.