By default, when a user first logs on to Outlook Web App, they are prompted to choose their language and time zone. Sometimes, it's required to change these settings for a certain user, or even, for all users. This can be done by an administrative user through Exchange Management Shell (EMS), and works for both Exchange 2010/2013 and Office365, with the cmdlet Set-MailboxRegionalConfiguration.
First, open EMS for your on-premise Exchange, or for Office365, connect to Exchange Online using Powershell.
You can then set/change the language and time zone for a user like so: