I say, don't bother. Use the default setting for your country, and allows users to change it if they want.
It's still on my to do list to write a plugin that would allow users to configure their timezone, preferred date and time formats, etc. For it to work properly, core would need to stop assuming any specific date formats though.
In any case, you can pass the start of the week setting to datepicker view with datepicker_opts (I think that's what we named it)
Have you tested with the latest version of Tidypics for Elgg 2.3? Even with version 2.3.1 of Tidypics Plus the popup won't open on my test site but I get an error instead ("Ajax view can't called directly" / "Page not found"). I don't know if this error is due to Tidypics Plus not working correctly together with the latest version of Tidypics or if something else fails to work for me when testing.
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