Prince of Persia: Sands of Time has Italian dubs but the GOG Store page doesn't list it. Is there a reason for this? Metal Gear Solid is another example.

Because said dubs / translations are likely outsourced and the property of third parties that may not be willing to negotiate a new contract / might want too much / may have folded / cannot be contacted etc etc.
Or to get them here its more work than the probable income generated by them.

Edit: also factor in most older games may not have included any licence for digital distribution at all in their original deals and would have likely had specific regional restrictions for use.

Not to mention that until rather recently, a lot of dubs (even English) stank like durian due to poor practices, restrictive content policies, and no small amount of executive meddling to sell media franchises to children which were made for adults.

Czech voice overs are also basically never included. All Prince of Persia games, The I of the Dragon, Warcraft 3 + FT, Disciples 2 just to name a few (out of quite a lot). Not only on GOG but in general. Back in the glory days of retail, the game voice acting scene over here was quite robust.

It probably has a reason, but hard to tell and it will differ from game to game. The legal jungle in this area is most likely simply too dense to cut through in a timely manner.

I once uploaded a backup copy of my Brazilian Carmageddon version for one of the Blue Guys from Gog. I think he wanted to have it in hands in case they managed to license that dub for distribution here, but, alas, it never materialized.

Though, honestly, the Brazilian dub was kind of a very lazy dub. So there's not a lot to miss there.

I found an Italian dub for Thimbleweed Park:
https://www.gugames.eu/twp-italian-voiceover/

Direct links to download the two patches:
main game patch
DLC patch

The Norwegian dub for The Longest Journey is not included in any digital copies. It's supposed to be really good too, so that's a shame. The sequel has it though.