When I logged in though Mission Control as a user with a specified backend language, that language was not being applied. When I logged in through BlueStork, it was being applied. And if I changed from BlueStork to Mission Control, that specified backend language stayed in place ...until I would login again with Mission Control.
So I suspected a bug with the login screen - and found it..
In the overriding file mod_login/default.php... I commented out the hidden input for "lang" (where the value was being set to an empty string). This seems to have solved the problem. I now can log in to Mission Control as a user with a specified backend language, and then MC uses that language.