Why are all games only in English??

ALL games?

http://www.gog.com/mix/multilanguage_games

They aren't. Several of them are available in other languages. Even some that don't specifically say so. (Many are only English though.)

As for a reason, licensing is one - often there's a different rights-holder to the translation than to the game itself.

Regarding the Daedalic games that are originally in German and the English translations are owned by the developer... you'd have to ask them yourself via, uh... or [url=https://twitter.com/daedalic]twitter?

The ones on GOG or games in general?

I'd expect the answere to be the same anyway: maximizing potential buyers compared to production cost.
Ofc that is a wild guess i do not have any more information than you on this.

The has now a language filter. Gog has added language files for some games (Broken Sword, Stronghold series) later, so they working on this. Beside the licensing problems (Whispered World, Dark Eye) it's also a question of support (testing) and preparing. It seems to me if gog gets multilingual versions or files from the publisher, they release them, but it's not on high priority for them (most customers here are native english-speaker). If you want a better multilingual support you maybe want to take a look on [url=http://www.dotemu.com]dotemu, they try to offer mulitilingual versions, including some games which are english-only here on gog.

Rule Brittania! Brittania rules the waves!
English never never will be under additional languages!

If you're reffering to Strike Suit Zero, then don't worry - the other languages have been added, and the gamecard shows this as well.

Because GOG wants people to learn English.

Whenever I hear someone mention Rule Britannia this starts playing in my head.
The dangers of first exposure, I guess ;)

Because my hovercraft is full of eels.

Well... french dubbed games are (often) subpar. Baldur's Gate, Mass Effect, Deus Ex: HR in french... yiiirk.

So, for me, games in english are a blessing from the heavens. Really.

Same here. I always try to play games in the original language if I can, and English if I can't.
The voice actors are typically much better in English, and you almost always lose some things in translations.

As for the reason GOG mostly provides English-only games, I'd guess it mostly has to do with licensing issues (since often games are published by different companies in different languages) and cost effectiveness.
It's likely that there simply aren't, for instance, potential customers who can only speak German to make getting the German version of games worth it. Same with French or just about any other language than English.

Indeed. Have been pirating games for this a lot, to find the english sound files for retail french versions (that don't state on the box whether they're multilangage or not). And reluctantly switched to digital downloads for that too, the ability to buy games with original voices (only localised versions are available in shops here). Heck, whenever I went in Greece, I was plundering the shops for english versions, as they seldom dub games in greek.

French versions are horrible. "French distributor's son's being asked to do all the voices during the weekend" horrible. That being said, french cartoons/animations dubbings are not much better.

Drop your panties, Sir William; I cannot wait until lunchtime