How to set culture specific alt text on an image

Hi guys, 

I have problem to change alt text property in the existing ImageFile class to culture specific.

I found this topic in episerver (https://world.episerver.com/forum/developer-forum/-Episerver-75-CMS/Thread-Container/2016/6/image-/)

I tried first to moidy the existing ImageFile and it didn't work. I created a new contenttype "LocalizedImageFile" and implement ILocalizable interface class instead.

[ContentType(GUID = "0A89E464-56D4-449F-AEA8-2BF774AB8745")]
    [MediaDescriptor(ExtensionString = "jpg,jpeg,jpe,ico,gif,bmp,png,pdf")]
    public class LocalizedImageFile : ImageData, ILocalizable
    {
        public IEnumerable<CultureInfo> ExistingLanguages { get; set; }
        public CultureInfo MasterLanguage { get; set; }
        [CultureSpecific(true)]
        public virtual string AlternateText { get; set; }
        public override Blob BinaryData
        {
            get
            {
                if (ContentReference.IsNullOrEmpty(this.ContentLink))
                {
                    return base.BinaryData;
                }
                if (this.Language.Name == this.MasterLanguage.Name)
                {
                    return base.BinaryData;
                }
                return this.LoadMasterContent<LocalizedImageFile>().BinaryData;
            }
            set
            {
                base.BinaryData = value;
            }
        }
        [ImageDescriptor(Height = 48, Width = 48)]
        public override Blob Thumbnail
        {
            get
            {
                if (ContentReference.IsNullOrEmpty(this.ContentLink))
                {
                    return null;
                }
                if (this.Language.Name == this.MasterLanguage.Name)
                {
                    return base.Thumbnail;
                }
                return this.LoadMasterContent<LocalizedImageFile>().Thumbnail;
            }
            set
            {
                base.Thumbnail = value;
            }
        }
    }
    public static class LocalizableMediaDataExtensions
    {
        public static T LoadMasterContent<T>(this LocalizedImageFile imageFile) where T : IContent
        {
            var contentRepository = ServiceLocator.Current.GetInstance<IContentRepository>();
            var content = contentRepository.Get<T>(imageFile.ContentLink.ToReferenceWithoutVersion(), new LanguageSelector(imageFile.MasterLanguage.Name));
            return content;
        }
    }

My question is what shall I do with the existing images which are "ImageFile" type?

I tried to upload a new picture and it still ImageFile type, not LocalizedImageFile.

Or any other tips to set culture specific alt text on an image?

Thanks,

ChiChing