download lib from https://bitbucket.org/Unity-Technologies/xcodeapi

xcodeapi.cs

[PostProcessBuild]
public static void OnPostProcessBuild (BuildTarget target, string path)
{
    string localeString = "zh-hant";
    AddLanguage (path, localeString);
}

static void AddLanguage (string path, params string[] languages)
{
    var plistPath = Path.Combine (path, "Info.plist");
    var plist = new PlistDocument ();
 
    plist.ReadFromFile (plistPath);
 
    var localizationKey = "CFBundleLocalizations";
 
    var localizations = plist.root.values
    .Where (kv => kv.Key == localizationKey)
    .Select (kv => kv.Value)
    .Cast<PlistElementArray> ()
    .FirstOrDefault ();
 
    if (localizations == null)
        localizations = plist.root.CreateArray (localizationKey);
 
    foreach (var language in languages) {
        if (localizations.values.Select (el => el.AsString ()).Contains (language) == false)
            localizations.AddString (language);
    }
     
    plist.WriteToFile (plistPath);
}