Improved UI language management

[florian]

At the moment all available language files are loaded, but just the user's interface language and English (for missing keys) are necessary.

Fix this:
​http://framework.zend.com/manual/en/zend.translate.sourcecreation.html
​http://framework.zend.com/manual/en/zend.translate.adapter.html

Problems:

• User#getUILanguages() uses an array with all languages from the translate adapter, such a list has to be created manually (and cache it?)
• Use individual cache tags for individual languages (option tag) and check if this is working correctly, because it's just enabled in production mode normally