Add locale in DatePickerThemeData

[MajdSallora]

This PR changes the date picker were add locale in DatePickerThemeData

List which issues are fixed by this PR. You must list at least one issue. An issue is not required if the PR fixes something trivial like a typo.
Fixes #148202

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[MitchellGoodwin]

Mostly LGTM with one nit. Thank you for putting together this fix.