There is Steam version and a Repack, does Repack contain mods? Then Steam version is a clear game with none?
Pretty much I think, but there is a S&B (Skull and Bones) Patch for Steam version so it could equal footing with Repack.
Being an "unDRMed" version there will be no options to participate/connect to any in-game community features?
There is none. This game 3 years old and the dev has moved on to the next title, Room Girl.
How bad (or maybe not) is MTL in the game?
Back then EN translation was done by a team, so it was not an MTL. But after Steam version came around, repack is using the translation data from it. So in Repack, you will get the same EN translation as Steam version. Not sure whether it's MTL or not, but Steam's translation are abit weird.
How much will I miss if I download base game (Steam version)? How does the game feel without mods?
I think the first thing you'll notice that is censorship? CMIIW, BepinEx plugin is the core of mods and plugins. So without that, you will not get uncensor, clothing, hair style, accessories and etc. Also limited
Could you explain slang that is commonly used in the game/community? (for exaple what are Cards)
Cards are character data bcs the character you made are saved in PNG format with portrait orientation. So it looks like a card.
Studio is the second executable/game you will find in the game directory other than the main game itself. In the main game, what you can do are making characters and use them to play the animations provided by the game. There are alot of position, but you can only choose from them. In sense, there is a limit of what you can do there.
Meanwhile in Studio, you can pretty much think it as "dress up game for adult". In there, you can have the character you made poses as you like in the map that does not exist in the main game and make some pinup. Or animate them to make custom animation that go beyond your wildest imagination.
If you are interested in the Studio, its better to go to the other
thread that more focused on Studio.