I have reverted using Galaxy and then backed up the pak, but as others pointed it out, not an ideal solution :).

Like you say, not an ideal solution as not everyone uses Galaxy so rolling back to get the pak file is not an option. This fix on Nexus is the only way to get the file if you exclusively use offline installers.

I do wish GOG found some way of sorting this out as it's really not a good look to have to go to an external website to grab a file to fix the game that GOG is selling in a broken state.

I mirrored the file:

https://www.mediafire.com/file/yy3adky1a50hcq1/Game.pak-118-1-0-1540446891.7z/file

md5sum:
bdc39e26c95b803ca5998aca66efb415 *Game.pak-118-1-0-1540446891.7z

sha1sum:
93a3bb8da7368bfa31ebf307ee061ba943243434 *Game.pak-118-1-0-1540446891.7z

<hugs>

Thank you, everyone. This is the kind of thing I always loved about gog.

Finally, after more than three years, it seems that this has been fixed. Larian "recently" released a new version that adds Brazilian-Portuguese localisation, which --- as a side effect --- also fixed the bug. The offline installers were updated today.

Nice to know its finally fixed. I have been seeing more developer update there GOG games lately. Sins of a solar empire got updated after so many years.

Yay, finally

That's incorrect information. Larian knew about this bug since at least December 2017 when I personally told them about it, and I also personally received acknowledgement back from them at that same time that it was indeed a bug.

So that's almost 5 years that passed while they knew the bug existed but during which they did not bother to fix it, not just three years only.

In fact, I just checked my email and I still have Larian's acknowledgement to me of the bug, which is dated 12/18/2017.

I am glad it's finally fixed after almost 5 years, but that is quite a ludicrous time frame to wait for a non-broken downloadable offline installer version of the game to become available to customers who were supposed to have had bought a working game at the time when they paid for it (which was in 2017 in my case).

I felt like I was being outright ripped-off during all of that time, since there was no way to download a non-broken version.

Although I had come to expect that Larian would never bother to fix it, so late is better than never, I guess (but I remain nevertheless still very disgruntled over the long period of time being stuck with a broken game that I paid for).

GOG should have stepped in back in 2017, and/or in all the years since, and started offering the non-bugged version of the offline installers to every customer who bought the game, by default.

That also would have solved the problem way before 5 years passed until Larian finally did, but instead GOG also did nothing about this problem for almost 5 years, which is a very bad look on GOG's part too, just like it is for Larian's part as well, IMO.