There comes a time that you as a administrator can get a request from the business to change the language of a specific or multiple mailboxes. This kind of request is even more likely if you are in the stage of migration to Microsoft 365 of just after migration.
As a user can simply change the language in the OWA environment you could create some documentation and share this with the business but for multiple and shared mailboxes this is not a preferred route.
As administrator of the Exchange Online environment connect to Exchange Online with Powershell using the Exchange Online Powershell V2 Module.
For more info over the Module : About the Exchange Online PowerShell V2 module | Microsoft Docs
Connect to Azure Online using the command : Connect-ExchangeOnline
Type the following to set the language for a specific mailbox
set-MailboxRegionalConfiguration -id Username -LocalizeDefaultFolderName:$true -Language nl-NL
Type the following to change the language for all user mailboxes
Get-Mailbox -RecipientTypeDetails UserMailbox -ResultSize Unlimited | set-MailboxRegionalConfiguration -LocalizeDefaultFolderName:$true -Language nl-NL
Run the following command to confirm that the above command worked. It will show the name of the user and the set language for the mailbox.
Get-Mailbox -RecipientTypeDetails UserMailbox -ResultSize Unlimited | get-MailboxRegionalConfiguration | ft identity,Language
If you don’t want to go trough all the above steps than you are in luck! I have written a little script with menu to do the job for you. I have included Dutch and English language to choose from.