The selector that chose the startup locale, or null when the chain fell through to the project locale.
Set during LocalizationSettings.InitializeAsync. Read it to tell whether a user override was applied at startup or
whether the locale came from the project default. Pair it with LocalizationSettings.EvaluateStartupLocale to show
both the chosen locale and the one that would have been chosen without the override.
Additional resources: LocalizationSettings.EvaluateStartupLocale, LocalizationSettings.StartupSelectors