iOS: Determine if device language is Right to Left (RTL)

There is an official way to do it:

if ([UIApplication sharedApplication].userInterfaceLayoutDirection == UIUserInterfaceLayoutDirectionRightToLeft) {
}

I would recommend against using some of the other solutions, because they will not always return the correct locale. Just because it's on the top of preferred languages doesn't mean that the application supports it.

Source: https://developer.apple.com/library/content/documentation/MacOSX/Conceptual/BPInternational/SupportingRight-To-LeftLanguages/SupportingRight-To-LeftLanguages.html

NSLocale has two methods +characterDirectionForLanguage: and +lineDirectionForLanguage:. The first is presumably Left-to-Right vs Right-to-Left and the second is Top-to-Bottom vs Bottom-to-Top. You can pass it the result of [[NSLocale currentLocale] objectForKey:NSLocaleLanguageCode].

Update:

The original question asked was how to determine whether the device language is RTL. Using +[NSLocale characterDirectionForLanguage:] and +[NSLocale lineDirectionForLanguage:] is unambiguously correct for that; you can pass either [[NSLocale currentLocale] objectForKey:NSLocaleLanguageCode] or [NSLocale preferredLanguages][0] to that to get the relevant info (I'm not sure offhand whether the NSLocaleLanguageCode uses the preferred language, or the set region).

However, it's very likely that what the original poster actually wanted to know is whether the application's interface should be laid out in RTL or LTR. This is very similar to asking what the direction of the language is, except it takes the application's available localizations into account. If the application is not localized into the user's preferred language, it will use a non-preferred language instead. And the answer to this question is to use [UIApplication sharedApplication].userInterfaceLayoutDirection.

In iOS 9 one can determine the current direction for each individual view.

if #available(iOS 9.0, *) {
  if UIView.userInterfaceLayoutDirection(
    for: myView.semanticContentAttribute) == .rightToLeft {

      // The view is shown in right-to-left mode right now.
  }
} else {
    // Use the previous technique
    if UIApplication.shared.userInterfaceLayoutDirection == .rightToLeft {

        // The app is in right-to-left mode
    }
}

This is the recommended way of determining the layout direction in iOS 9.

WWDC 2015 video New UIKit Support for International User Interfaces. After minute 31:20.