You can set the Localization for an application in SiteAdmin.
1. Login to SiteAdmin
2. Click on application
3. Change settings under Localization.
Individual users can override the applications Localization from the Settings menu in the application.
And finally under SiteAdmin -> Settings -> Localization you can set default values that are used for new created applications.
* There is a useful feature in SiteAdmin to login "impersonated". In the application list in SiteAdmin click on the icon left to the name of the application, then select the username to login as. In this way you can verify that a user gets the proper experience when logged in.