There are three language settings that you can configure for Outlook Web App.

• The sign-in and error language setting applies to individual Outlook Web App virtual directories. The sign-in and error language is the language that will be used for errors and the forms-based authentication sign-in page. If a value isn't set for this language, the default value is 0. This means that the default sign-in and error language isn't defined. If the sign-in and error language isn't defined, Outlook Web App will default first to the language set on the Web browser on the client computer. If the language set on the Web browser on the client computer isn't supported by Outlook Web App, Outlook Web App will use the language of the Client Access server.
  
  
• The default client language setting applies to individual Outlook Web App virtual directories. The default client language is the client language that's used by Outlook Web App unless the user uses Regional Settings in Outlook Web App to change the language and time zone. The default value for this setting is 0. This means the default client language isn't defined. If the default client language isn't defined, users will be prompted to choose a language and time zone the first time that they sign in to Outlook Web App. If the default client language value is defined, users won't be prompted to choose a language and the Outlook Web App time zone will use the time zone of the Client Access server. Defining the default client language causes the default folders to be renamed based on the specified language. Users can change the client language and time zone by using Regional Settings in Outlook Web App and can rename the default folders after they sign in.
  
  
• The client languages are set on individual mailboxes and affect the language that's used in Outlook and Outlook Web App. If multiple languages are configured, the first language in the list that's supported by the Web browser will be used. If none of the languages in the default languages list is supported by the Web browser, the Client Access server language will be used.