When a user updates the value for the Language field under the User Profile page on service portal this same change does not reflect on the Language field (preferred_language) field on the sys_user record.

1. Log in as a user and navigate to service portal (i.e. instancename.service-now.com/sp)
2. Click on the Profile located at top right corner of the page and select "Profile"
3. Under the User preferences section of this page change the value of the Language field to any available language (i.e. Spanish or French). Make note of the value selected.
4. Now as an admin or a user who has access to the sys_user form outside of the portal navigate to the same sys_user record
5. Notice now that the Language field (preferred_language) on this form does not have the same value as the one selected earlier on the portal

The workaround at this point would be to add the "preferred_language" field from sys_user form to the user profile page on the portal. To do this:

1) Navigate to any sys_user form
2) Switch the form view to "Service Portal"
3) Add in the field Language (preferred_language) to this form view
4) Save the change

This field should now show up under the User Profile page on the portal. The user can set the value on this field and that would save the change to the preferred_language field on sys_user.
Please note though, however, that that is not the same as the other "Language" field under the "User Preferences" section of the User Profile page on the portal. That other "Language" field sets the "user.language" user preference instead. This user preference is the one that controls which language the user should be seeing while on both the portal and normal platform UI.