Since when we - the italians - have become so irrelevant in the videogame market?
Since always (and don't get me wrong. I'm Italian as well as you).
Italy has traditionally been an unattractive market for RPG developers.
I strongly disagree. I remember the RPG market being a flourishing one in our country, at least up to a decade ago.
Many important RPG titles had an official italian translation (at launch or added later on, but NOT provided by fans), an italian manual and in some cases even italian dubbing!
This was true for the epic PC series (Baldur's Gate original saga, while italian was ignored for the EE), the Blizzard titles (Diablo, Warcraft), as well as the console ones (Final Fantasy comes to mind).
German/French/Spanish/Italian was a standard for the europe market at the end of the 90s/early 00s.
I'm totally fine with the first D:OS not having italian at launch, but after its success here in our country and the free translation provided by fans and integrated via patch, now I'm VERY surprised that Larian didn't bother to give us one for the second title. It seems out of touch for a company so lovable and friendly like Larian. Are they counting on the fans again?
I started playing CRPGs with Ultima IV. My list of true epic Computer RPGs includes names like Ultima VII, Ultima Underworld, Darklands, Betrayal at Krondor, Dungeon Master, Pool of Radiance, etc...
Believe me: when I say that traditionally Italy has never been an appealing market for RPG developers I have PLENTY of evidences of that. For a Baldur's Gate saga that was translated in Italian you have a Fallout saga (the first two chapters + Tactics) that can be played in Italian only because of Ragfox. For a translated Diablo you have an untranslated Planescape Torment. Troika games have never been officially translated and so on so forth...
Our language has never been a standard for this genre. Only AAA RPGs with really big budgets don't neglect Italy by default.
I also disagree about the Italian language in general: it is much more spoken and known than you might think, and still in the top 20 most spoken languages in the world (other than being one of the most beautiful ever, of course

).
If you consider that Divinity is a western-type RPG, you have to cut out some unusual markets/localizations from the list (like Hindi, Arabic, Korean etc.), so in the end Italian ends up way higher, easily in the top 10 most required languages for this kind of games.
Obsidian followed a similar route with Pillars of Eternity, and in the end they decided to add an official italian translation to the game after a survey on their forums. Since D:OS2 will appeal to a similar market, wouldn't it be a smart choice for Larian as well?
I have a degree in linguistics. So, I'm perfect aware that our language is spoken in many different countries outside of Italy. This doesn't mean, though, that you can reach new markets (outside of Italy) by including an Italian translation in your game. Second generation Italian Americans won't play your game in Italian, and the same goes for the descendants of our people all around the world...
The other languages I've mentioned, on the other hand, have a way more pronounced commercial appealing. For obvious reasons.