1 - Where are you making your Dashboard changes, on your own personal dashboard, or on the Site Admin - Appearance - Default Dashboard page (https://yourmoodle.com/my/indexsys.php) -- if the latter, then make sure when you're done making your change, click the "Reset Dashboard for all Users" button to force your change out to all users.
2 - Where are you assigning the role of Japanese student vs English student? At the site-level's Assign Roles page (https://yourmoodle.com/admin/roles/assign.php?contextid=1)?
3 - Your test student has a specific username & password that you're using to log in directly as them correct?
4- Can you verify your test student is assigned only one of these 2 roles and that they don't have any other site-level role.
Other than that, I'm not sure what else to suggest since your steps seem correct to me.
So see what other roles a user is assigned, go to their Moodle profile and click on the Preferences link on the next page, click on "this users role assignments"
Then it will list their roles as the site context followed by their roles in each course
I noted that removing the permission for both authenticated user and guest was key.
Wow! Excellent detective work!
I already had that permission off for a different reason and never would have connected that permission to this issue. Glad it's sorted!