My problem isn't the english language, but the fact that if a game EXIST on my language, I'd like to own it that way.
To the Moon has italian subs too, so why Gog doesn't update its version, including italian text?

I, for one, would not want to own a game localized into Finnish, and one of my friends is livid for not being able to change the language on Halo 3 (or maybe he doesn't know how to). Whatever floats yours, I suppose.

However, it's not a simple matter of "updating the version". My guess is that GOG hasn't got the localized version for one reason or another; most likely the contract with the developer only included the English version, and GOG would have to obtain (=buy) the rights to the localizations separately. Maybe they struck a bad deal, maybe they figured it wasn't worth the money, maybe maybe maybe. It's a simpler matter than with old games, where the people who own the rights to translations may be dead, indifferent or difficult to reach out to, but that doesn't mean that GOG cares enough to go through with it.

By the way, the forum is a relatively ineffective way of contacting GOG. You might want to pester them through the Contact Us form at the bottom.

I bought To the Moon on Gamersgate, but I only received a download link to the English version. With the trick I posted above it was possible getting a German version, but it's not offered by the store itself.

In fact, Gamersgate does not even list what languages one can expect when buying. A game using Steam is the only thing because you can lookup on Steam what languages are to be expceted. Otherwise, just guessing (except English). In this point GOG is at least honest.

Until further investigation, I was proved wrong. The game is multilingual only on Steam, it seems.

the argument still stands for many other games, though.

I would say three, the third being "lack of Linux support". Otherwise yes, it's perfect.

Oh, yes, you are right:
Supporting monopolists only (M$ & Apple) can never be right.