Added basic language support for sending out invitations:

Added basic language support for sending out invitations:
- This allows for example sending German mails from an English instance
- Added some checked type-hints (all around language stuff)

Some code improvement:
- array $foo=null and then an is_null($foo) makes no sense,
  better use array $foo=array() directly.
- onTitle()'s parameter $paths now always must be an array

Signed-off-by: Roland Haeder <roland@mxchange.org>