NSDateFormatter and current language in iOS11

It appears that default behavior for NSDateFormatter has been changed in iOS11. This code used to work and produced date formatter according to currently selected iPhone/iPad language prior to iOS11:

 _dateFormatterInstance = [[NSDateFormatter alloc] init];
 _dateFormatterInstance.timeZone = [NSTimeZone systemTimeZone];

Looks like in iOS11 we have to explicitly specify locale property for it:

 _dateFormatterInstance = [[NSDateFormatter alloc] init];
 _dateFormatterInstance.timeZone = [NSTimeZone systemTimeZone];
 _dateFormatterInstance.locale = [NSLocale localeWithLocaleIdentifier:[[NSLocale preferredLanguages] firstObject]];

Can somebody confirm my findings?

This isn't a problem with NSDateFormatter, it's a change in how iOS 11 supports localization.

Under iOS 11, [NSLocale currentLocale] only returns languages supported by your app's localizations. If your app only supports English (as the base localization), then no matter what language the user selects on the device, currentLocale will always return English.

Under iOS 10 and earlier, currentLocale would directly represent the user's chosen language and region, regardless of what localizations your app supports.

Classes such as NSDateFormatter default to using NSLocale currentLocale. So no matter what language your app actually supported through its localization, classes like NSDateFormatter would show text in the language set on the device, even it was different from the language being used by your app.

iOS 11 fixes this inconsistency. While one could argue that this change breaks lots of apps that only support one (or just a few) language, it actually makes the app more consistent.

To make all of this clear, consider an example. You create a simple test app with a base localization in English. If you run your app with iOS 10 and the device's language is set to English, you obviously see English text and you see dates formatted for English. If you now change the device's language to French and restart the app, the user now sees English text in the app (since that is its only localization) but dates now show with French month and weekday names.

Now run the same app under iOS 11. As with iOS 10, if the device's language is English you see everything in English. If you then change the device's language to French and run the app, iOS 11 sees that your app only supports English and currentLocale returns English, not French. So now the user sees English text (due to the app's localization) and dates are now also still in English.

This actually appears to be more of a bug than an intentional change in behaviour in iOS 11. If you only have one language set, this behaviour isn't present as Locale.current always returns the correct language and region even if your app isn't localized to that language.

However, if you have more than one language - such as French and English - then iOS 11 appears to always favour English or the closest supported language in your app when using Locale.current.

Locale.preferredLanguages appears to return the correct language-region information, so you might be able to use that instead.

Below is an example showing the output from Locale.current and Locale.preferredLanguages, showing the inconsistencies.

This was generated from an app that only supported English. On the device, French was set as both the primary language and region, with English (Australia) set as a secondary language in the first example.

(Incorrect) Locale.current with multiple languages - note how English is the language, when it should be French and therefore fr_FR

  - identifier : "en_FR"
  - kind : "current"

(Correct) Locale.preferredLanguages with multiple languages

  - 0 : "fr-FR"
  - 1 : "en-AU"

(Correct) Locale.current with French as the only language

  - 0 : "fr-FR"

(Correct) Locale.preferredLanguages with French as the only language

  - identifier : "fr_FR"
  - kind : "current"

Yes, the default behaviour is changed in iOS11 exactly as @rmaddy described.

In my case, I have a project with a base development language set to English but, on iOS11, when I changed the device's language to any other language (say Swedish) the dates would still be displayed as, for instance, Monday 6 November. This happened because my app didn't support any localization.

The solution was simple: in order to have the app displaying the dates in Swedish I just had to add an empty Strings.strings file and then, in projects settings, I added the Swedish localization. Although the strings file is empty, the app then became localized in Swedish so by changing the language, in Settings, to Swedish, we could see the same date as måndag 6 november, thus achieving the desired use-case of iOS10.

Note: if you do something like this and it doesn't work for you, when adding a language in Project Settings make sure to go to "Other" and pick a language from there (instead of just choosing one from the first-level dropdown).