Did this ever get a non-Russian release? All I can find on the internet is a completely Russian version. I know that the original game did have an English release because I played it (and it is even on Steam), but regarding the Mission Pack, it doesn't seem so.

I've wanted to play it for a long time after liking the main game a lot, but as all the in-game text is Russian, it makes it pretty difficult to know what to do in the levels, objectives and such. It is pretty hard to find any info about the game at all and besides a couple of Russian videos on Youtube, I couldn't find anything.

Am I correct in the assumption that it was a only ever released in Russia?

BTW, the games were made by Maddox Games, which has been owned by 1C Entertainment since 1999. There is even one game on GOG from Maddox - IL-2 Sturmovik: 1946. Would it be possible to get at least the base Z.A.R. game here? There are many 1C games on GOG and I get the feeling it's not here mainly because of how unknown this game is, not because it would be impossible.

EDIT:
Wishlist entries:
https://www.gog.com/wishlist/games/zar
https://www.gog.com/wishlist/games/zar_and_mission_pack

The Mission Pack seems to be Russian-exclusive, indeed.

As for the game itself, it was released years ago on Steam by Night Dive, alongside another Maddox-developed game, Madspace. If I had to take a guess, GOG likely prioritized other releases and completely forgotten about them afterwards.

Nightdive Studios is the current owner of Z.A.R. and MadSpace (another FPS from Maddox Games).

I think you should ask them why they released these games on Steam but not on GOG.

By the way, you can ask them about it right here:
https://www.gog.com/u/NightDiveStudios

But I don't know if this account is still active.

Poke em on twitter, I have usually gotten a response or start asking questions on their discord.

Z.A.R. Mission Pack was made for Russian language only, however it does have a bit of English text here and there, it seems they were going to release an international version but something went wrong.

I would suggest to poke them at Discord:
https://discord.com/invite/nightdivestudios

Grandpa doesn't have Twitter. (:

But maybe idbeholdME does.

well this grandpa does have twitter lol