Localization is feature provided by iOS which translates your app into multiple language.For Localisation,Internationalization is necessary.Internationalization is process of making iOS app able to adapt different culture,language and regions.

Localization in iOS

Create an individual Localizable.strings file for each language. The right side would be different for each language. Think of it as a key-value pair:

"str" = "str-language";

Access str in Objective-C:

//Try to provide description on the localized string to be able to create a proper documentation if needed
NSString *str = NSLocalizedString(@"string", @"description of the string");

Access str in Swift:

let str = NSLocalizedString("string", comment: "language");