Your default language is the language the application will display when you first enter, and can be set for each user in the 'People Maintenance' screen on the 'Employee details' tab. This is set to 'Default Language' (Workbench Default Text) by default.
This default can be set by the users themselves in one of the following ways:
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To set your default language from the 'Who Am I' screen, open the 'Who Am I' Screen from the 'Help' menu, confirm or set the Session Language to the language you wish to be your default, and click the button 'Set this Language as My Default'.
In either case you will get a confirmation message similar to the one shown here.