On user registration and admin pages user name min & max lengths displaying text 'CHARACTERS_XY' instead of values

User registration and user admin pages displays the text 'Length must be between [n] and [m].'  adjacent to the text box for user name, where [n] and [m] are the values configured for minimum and maximum name length. This displayed correctly in my phpbb 3.3.10 installation.

In my upgraded 3.3.11 installation that text instead displays as 'Length must be between CHARACTERS_XY and CHARACTERS_XY.'

See attached screen shots.