Access iOS dictionary programmatically

There is no API to do what you are asking.

There is an API for bringing up the built-in dictionary UI (not the original question but raised in some of the responses), including checking for whether a given word is defined:

if ([UIReferenceLibraryViewController dictionaryHasDefinitionForTerm:@"word"]) {
    UIReferenceLibraryViewController* ref = 
        [[UIReferenceLibraryViewController alloc] initWithTerm:@"word"];
    [currentViewController presentViewController:ref animated:YES completion:nil];
}

I don't think there is a public API to access the embedded Dictionary. The closest is using UITextChecker but that may not satisfy your needs

As an alternative, you could find and embed a Word Dictionary in the Language you want? It may require some extra work

There is no API, but if all you need is a list of words you can read it from /usr/share/dict/words:

    NSString *allTheWords = [NSString stringWithContentsOfFile: @"/usr/share/dict/words"
                                                      encoding: NSUTF8StringEncoding
                                                         error: nil];
    for (NSString *line in [allTheWords componentsSeparatedByString:@"\n"]) {
        NSLog(@"%@",line);
    }

If you need an arbitrary list of words you can get it from several online resources, here is one. The README at /usr/share/dict/README says iOS word list uses ftp://ftp.ox.ac.uk/pub/wordlists

For instant search you'll need to load the words on a structure suitable for prefix search, like a trie. Here is a sample project where I did just that.

If you need a dictionary, get one online, then include a pointer to the word entry in the trie node. If loading takes too much time you can experiment loading on the background, or compressing, or serializing the trie.