PreferredLanguage property value (in Microsoft portal) is set to en_US for all users created by
CCP > Office 365 > Users > Add New User button, despite the customer account locale or language defined.
Software-related issue #APSA-18354: "Propagate PreferredLanguage for users from OAP CCP to Microsoft Portal".
The issue was fixed in version 17.2. Consider an upgrate to the newest version of the APS Application to permanently fix it.