How do I set the language code for rendering in entity_view()?
I'd recommend looking inside of user_view() https://api.drupal.org/api/drupal/modules!user!user.module/function/user_view/7

That function should be called by entity_view() and it contains a drupal_alter() call. Print out the $build variable that goes into drupal_alter(). Is it using Japanese? Also print it out after the drupal_alter(). Is $build now back to English? If so, some other module is altering the display.