Technical issues with Heroes III.

I installed the translation for the complete Russian collection, and now everything works; before that, only the megapack was installed. Does this mean that the megapack is impossible to install?

I don't know; everything works fine for me with the Megapack (although I didn't adjust the animation speed). Perhaps the complete collection you have contains some "unnecessary" files that were "removed" (this is typical for repacks), which is causing the problems.