As you know, GOG DVD version doesn't have subtitles. I wonder if with HD Video Pack, is there a way to get subtitles turned back on? If not, then I sincerely hope there is a fan remake that will correct this issue someday. I can't play WC4 DVD version, and I have DOS version from GOG, but it doesn't play past mission 5 for whatever reasons.
I'm glad to be able to let you know that WCIV: Remastered will have subtitles throughout :^)
I'm afraid the HD pack doesn't re-enable subtitles in the base game - they're encoded within game files, rather than the videos themselves. However, if the issue is one of language, the HD packs are available in French and German, if that helps?