The default messages 'restricted_content_message' and 'restricted_content_message_anon' make use of the t() function allowing them to be translated. But if you supply custom messages through admin interface, those get saved as variables and retrieved using variable_get(). However, they do not get a t() function applied to them, so they can't be translated.

See restricted_content_variables() (line 168) and restricted_content_var() (line 178) in restricted_content.module for details.