After creating a new mailbox the first time a user logs on using OWA he has to set the language and the timezone. You can set this during provisioning of the mailbox, but it can only be achieved using the Exchange Management Shell.
To create a new mailbox (using the Shell) you can use something like this:
$password = Read-Host "1StrongPassword" -AsSecureString
New-Mailbox -Name "Joe Sixpack" -UserPrincipalName Joe@contoso.com -Alias Joe -Database "Mailbox Database 123" -OrganizationalUnit "contoso.local/accounts/sales" -FirstName Joe -LastName Sixpack -DisplayName "Joe Sixpack III" -Password $password -ResetPasswordOnNextLogon $true
Set-MailboxRegionalConfiguration -identity "Joe Sixpack" -language "nl-NL" -TimeZone "GMT+1"
The timezone is the timezone being used on the Exchange Server, so if this Exchange Server is located in Holland, and Joe Sixpacks location is in Holland as well there's no need to change the timezone.
To change the language settings on a Virtual Directory level you can use the following commands:
Set-OwaVirtualDirectory -identity "Owa (Default Web Site)" -LogonAndErrorLanguage 1033
Set-OwaVirtualDirectory -identity "Owa (Default Web Site)" -DefaultClientLanguage 1033