When a new Exchange Online mailbox is created the user is prompted to set language and time zone at first logon. This information is possible to pre-populate to make the experience smoother for the end users. This is particularly useful in a migration scenario.
These settings are set with the cmdlet Set-MailboxRegionalConfiguration.
Set-MailboxRegionalConfiguration -identity "firstname.lastname@example.org" ` -TimeZone "W. Europe Standard Time" -Language "sv-SE" -confirm:$false
Some useful links: