Get Mailbox Regional Config for one user:
Get-MailboxRegionalConfiguration -Identity "User1"
Get Mailbox Regional Config for all users:
Get-Mailbox | Get-MailboxRegionalConfiguration | Format-Table -AutoSize
Set Mailbox Regional Config for one user:
Set-MailboxRegionalConfiguration -Identity "User1" -Language nl-nl -LocalizeDefaultFolderName -TimeZone "W. Europe Standard Time"
Set Mailbox Regional Config for all users:
Get-Mailbox -Resultsize unlimited | Set-MailboxRegionalConfiguration -Language nl-nl -LocalizeDefaultFolderName -TimeZone "W. Europe Standard Time"