Sitecore does not implement hard validation on the language write access at field level, so to make sure a local editor would never be able to mingle with a language he did not have access to, we added the restriction code to the Item:Saving event queue p

gistfile1.cs

public class EnsureLanguageWriteAccess
    {
        protected void OnItemSaving(object sender, EventArgs args)
        {
            Item item = Event.ExtractParameter(args, 0) as Item;
            Error.AssertNotNull(item, "No item in parameters");
 
            var itemlang = item.Database.GetItem(string.Format("/sitecore/system/Languages/{0}", item.Language));
            if (itemlang != null && Sitecore.Security.AccessControl.AuthorizationManager.IsDenied(itemlang
                                                            , Sitecore.Security.AccessControl.AccessRight.LanguageWrite
                                                            , Sitecore.Context.User))
            {
                string loggedErrorMessage = string.Format("The user {0} has tried to edit a language {1} where AccessRight.LanguageWrite is denied. (item: {2})", Sitecore.Context.GetUserName(), item.Language.Name, item.Paths.ContentPath);
                Log.Error(loggedErrorMessage, typeof(EnsureLanguageWriteAccess));
                throw new Exception("You do not have access to write content in this Language context.");
            }
        }
    }