How to set CultureInfo for unit tests?

Want to test following code with different a culture other than the default:

public string Now =>
    DateTime.Now.ToString("g", new CultureInfo(CultureInfo.CurrentUICulture.Name));

So far,

[UITestMethod]
public void UI_NowIsLocalizable()
{
    // Arrange
    Clock clock = new Clock();
    Windows.Globalization.ApplicationLanguages.PrimaryLanguageOverride = "de-DE";
    string expected = DateTime.Now.ToString("g");

    // Act
    string actual = clock.Now;

    // Assert
    Assert.AreEqual(expected, actual);
}

While debugging the test, CultureInfo.CurrentUICulture.Name still shows us-en and the test passes (should fail).