Problem with routes if Controller is in a subfolder

I think it's exactly what I did just below... My sub-folder is 'admin', my controller 'Translations' and my method 'translate':

It doesn't work like this :

Code:

$routes->match(['get','post'], '/admin/translations/(:alpha)', 'admin/Translations::translate/$1');

I put the name of the sub-folder in the route, but when passing arguments, it doesn't work and the controller or method is not found ! 

Or should I put a "/" instead "::" between controller and method  ?
$routes->match(['get','post'], '/admin/translations/(: segment)', 'admin/Translations/translate/$1');