1 - Go to https://portal.office.com/ and enter your credentials (Email and Password).
2- Then click Mail.
3- Click the gear on the top right.
4- Then, in the tab to the right (Your app settings) click Mail.
5- In the left tab "General" select Region and time zone.
6- Choose the language of your choice and do not forget to check "Rename default folders so that their names match the language specified".
7- Click on save. You can log out of your Office 365. For the changes to apply, close your Outlook application and re-opened it.