I've heard complaints before (although rarely) that some DVD subtitles go by too fast to read.
If a foreign language were very info-dense for its pronuciation, an accurate translated form might go by too fast for some.
A solution is to offer multiple reading levels for any particular language. I
imagine two levels 'simple' and 'fully accurate' would suffice.
Then slower readers could read the good-enough for the storyline translation, and faster readers could see an accurate, completely presented translation.
This could be done without having to modify the DVD format or the language codes. Both or all the subtitle versions could just use the language name as is done today without changes. For example, a Japanese film is viewed. Subtitle 1 might be tagged 'English' but would be the simple form, subtitle 2 would also be tagged 'English' but might be the precise if longer form. The 'Languages' or 'Audio' menu on the disk could offer more info about each subtitle section.