Good practices for multiple language data in Core Data

I've done something similar to Shortseller, but without the use of categories.

InternationalBook and LocalizedBook are both custom managed objects with a one-to-many relationship (one international book to many localised books).

In the implementation of InternationalBook, I've added a custom accessor for title:

- (NSString *)title {
    [self willAccessValueForKey:@"title"];
    NSString *locTitle = nil;
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"locale==%@", [DataManager localeString]];
    NSSet *localizedSet = [self.localizedBook filteredSetUsingPredicate:predicate];
    if ([localizedSet count] > 0) {
        locTitle = [[localizedSet valueForKey:@"localizedTitle"] anyObject];
    }
    [self didAccessValueForKey:@"title"];
    return locTitle;
}

[DataManager localeString] is a class method which returns the user's language and country code: en_US, fr_FR, etc. See documentation on NSLocale for details.

See the "Custom Attribute and To-One Relationship Accessor Methods" section of the Core Data Programming Guide for an explanation of willAccessValueForKey: and didAccessValueForKey:.

When populating the data, I grab a string representing the user's current locale ([DataManager localeString]), and store that along the with localised book title in a new LocalizedBook object. Each LocalizedBook instance is added to an NSMutableSet, which represents the one-to-many relationship.

NSMutableSet *bookLocalizations = [internationalBook mutableSetValueForKey:@"localizedBook"]; // internationalBook is an instance of InternationalBook
// set the values for locale and localizedTitle
LocalizedBook *localizedBook = (LocalizedBook *)[NSEntityDescription insertnNewObjectEntityForName:@"LocalizedBook" inManagedObjectContext:self.bookMOC];
localizedBook.locale = [DataManager localeString];
localizedBook.localizedTitle = theLocalizedTitle; // assume theLocalizedTitle has been defined.
[bookLocalizations addObject:localizedBook];
[bookLocalizations setValue:localizedBook forKey:@"localizedBook"];

Since the localised titles are being stored in the LocalizedBook managed object, you can make the title attribute a transient, but if you do that you can't use title in a predicate.

The nice thing about this approach is that the implementation of the to-many relationship is transparent to any consumers. You simply request internationalBook.title and the custom accessor returns the appropriate value based on the user's locale behind the scenes.