If I remember right, subtitles in the DVD version of WC4 don't actually work. They are integrated into the game's code, but do not display in the game. It may be possible for someone to fix this at some point, but nobody has made a patch like that yet.
I think the problem is that subtitles are not included in the VOB files (no DVD subtitles) In many DVD movies the up and down black zone are present, but subtitles are 'printed' over the movie picture.
Launch the game with the 'wc4dvd.exe -l2' command line allow you to have french subtitles in the dialog choices, in in-flight scenes and it also translate the interface.
same command with -l1 for german translation.
The DVD version uses the same command line options than the Win95 Cd version (or win95 patched CD version) but i don't know if the -no_interlace and the -true_color commands have any effect.
for the movies subtitles, if someone could extract subtitles with timings from the game code, i could try to rebuild the Vob files including subs.
There should be 3 languages : english, german and french.
Problem will be : how activate the subtitles when included in the VOB?. The game has no right-click DVD options like a video DVD-player