[MRTCore] Add global PrimaryLanguageOverride switch

[stefansjfw]

Changing the application language by setting Windows.Globalization.ApplicationLanguages.PrimaryLanguageOverride to a specific language is not supported for WASDK unpackaged app, only for packaged ones. To support the same behavior for unpackaged applications similar switch is introduced into WASDK MRTCore module.

Sample app used to verify the change: https://github.com/stefansjfw/testwasdk/blob/master/testlocalwinappsdk/App.xaml.cs#L36

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[stefansjfw]

/azp run

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[crutkas]

while doubtful /azp run

[crutkas]

/azp run

[azure-pipelines[bot]]

Commenter does not have sufficient privileges for PR 4181 in repo microsoft/WindowsAppSDK

[crutkas]

This is needed for PowerToys to fully enable language override

[stefansjfw]

note: changing namespace to Microsoft.Windows.Globalization.ApplicationLanguages is in progress (as decided during API review)

[stefansjfw]

note: changing namespace to Microsoft.Windows.Globalization.ApplicationLanguages is in progress (as decided during API review)

change pushed

[kmahone]

/azp run

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[kmahone]

This has been merged as #4523