Fix notices when key does not exist

Do an early detection when a key does not exist, to prevent notices
generated by the rest of the function.
Multi-language aware controller class

Added a basic controller that uses different view scripts for specified
actions, based on the currently chosen language. The view scripts will
therefore have the format <action>-<language>.phtml. If the view script
file for the given language is not found, it will fall back on the
default language.