On iOS Bundle.main.preferredLocalizations returns the list of languages the application bundle supports in user-preferred order with the first element being the language the application is running in. Additionally Locale.preferredLanguages returns the list of languages in the order they are presented in Preferences.app > General > Language & Region > Preferred Languages with the first element being the user's "primary language" (i.e. the language the system is running in). However this only seems to be true unless the user has chosen a per-app language which is different from the primary language in which case Locale.preferredLanguages.first is equal to Bundle.main.preferredLocalizations.first - regardless of the latter's position in the Preferred Languages list. Furthermore this seems to change depending on the value of the "AppleLanguages" key in the User Defaults' global domain (see c.f. https://stackoverflow.com/a/42648166). Is this behaviour documented anywhere? Addendum: I know that according to https://forums.developer.apple.com/forums/thread/718512?answerId=733680022#733680022 AppleLanguages is an implementation detail, not something that’s considered API. Locale.preferredLanguages is API, though.

Is there a way to view the opt-in rate via the API? I have been unsuccessful in finding one.

I am trying to retrieve the numbers displayed in "App Analytics" under "Metrics" > "Active Devices" using the API. Since there does not seem to be a dedicated report for this metric I am using the daily "App Sessions Standard" report and by looking at the "Sessions" field I am able to recreate the numbers displayed for the "Sessions" metric. However if I try to infer the active devices by looking at the "Unique Devices" field my numbers for individual days are in the same ballpark (deviations may be attributable to privacy reasons?), but the total number of devices is way off. Is there a better way to recreate this metric?