As you know, end-users language and regional settings are very important when working with Office 365 as it defined lot of things – from correct working hours to date and time display.
Well, end-users will be able to manage their language and regional settings from the Azure AD My Account (https://myworkaccount.microsoft.com/)
This option will be available to all users but for end-users working in hybrid mode (aka with directory synchronization in place) will not be able to change their primary language but will be able to add additional language.
You can instruct your end-users to use the My Account portal (https://myworkaccount.microsoft.com/) to manage these settings by accessing the Settings & Privacy blade
They also manage their language and regional settings (only for cloud only users) from any Office 365 application when using a web browser when opening the gear (aka Settings) menu, available at the top right near their picture