Regarding concealed doors and problems to be solved by the player rather than the character, one of my Tunnels & Trolls games involved handing the players one of those little sliding tile puzzles, onto which I had stuck a drawing of the door. The scrambled image was exactly what their characters could see on the wall and the door could only be opened when the puzzle was solved.
Since I detest dealing with puzzle rooms myself I tend not to pull this sort of stunt these days.
Concerning language and introducing it to players, I rather like the method Professor Barker used, especially in his novels, of combining a Tsolyáni word with an English one to provide immediate context and familiarity. Thus we have references to Dná-flour, Dná-grain beer, tiny Drí-ants, a Hmélu-calf and so on. You may not be able to identify Dná-grain and know how it differs from another type, but as a player you instantly grasp roughly what is being discussed. It lets you pepper the conversation (or spice it up with hot Hlíng-seed, if you prefer) without everyone struggling to remember what the words mean.
One of the minor revelations on my path away from dungeon-bashing was that telling the players to solve a puzzle may be fine as a game, but it’s not at all a simulation: we may be able to determine how good each character is at the 15-puzzle, but we can’t tune our own abilities to match.