What free RPG or what non-English RPG did you enjoy most? That's easy: Old School Hack (OSH) by Kirin Robinson. Old School Hack is absolutely brilliant. It is a superb mix of Old D&D elements with modern game design. You get all the flavor of the early editions of D&D but with a very streamlined and flexible set of rules. Not only that but the rules, character sheets, and playing aids are all included in a free pdf of under 30 pages. Along with OSH I would include the expanded version of it, Fictive Hack, by Andrew Shields. I heartily recommend you check out both games.
I ran two sessions of this which were mini dungeon adventures. The rules were a pleasure to play, both for the DM and the players. Later, we adopted the Awesome Point rules for our Castles & Crusades game.