It looks like it’s only possible, in the project properties, to set whether the locale dropdown selector will be shown or hidden. Even when it’s set to be shown, if autologin is used users won’t see that, so it should be possible to to set a default locale for a project in the Vision project properties.
You could always just use a client startup script and system.util.setLocale - probably easier, and certainly faster, than waiting for us to add a project property 
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