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");