For some strange reason, it not listing the last element of the rcube_languages array.
It may be a bug. I fixed adding an extra element at the end of the array.
For example:
$rcube_languages = array(
'en_US' => 'English (US)',
'en_GB' => 'English (GB)',
'es' => 'Español',
'nn' => 'Defaul Language'
);
$rcube_language_aliases = array(
'en' => 'en_US',
'nn' => 'en_US'
);