Fixing JHtmlContentLanguage::existing()

JHtmlContentLanguage::existing() uses a cached `static::$items` to return the list of installed languages. However this doesn't take into account the option for the `$all` and `translate` arguments of the function.
The first call determines what will be returned and subsequent calls always return the same, ignoring the passed arguments.

You can test this in com_content where the language batch function does miss the `All` option because the Search Tools are calling the function prior to the language batch.
This PR will solve that and restore the expected behavior.

See tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=32846
Previous PR for reference: https://github.com/joomla/joomla-cms/pull/2621