When I used to run my own scenarios years ago, I occasionally would describe something like "you hear goblins chattering about something behind the door." Inevitably, the players would ask, "Do I know how to speak Goblin?" My answer was to have them roll their current number of mind points on d6s. If any 6s came up on the dice the answer was yes and I would tell them what the Goblins were saying. I would also have them mark on their sheet whether they knew the language and the answer was permanent.
Why use current mind points and not starting points? I used to say that since mental attacks lowered your mind by screwing with your brain, it is possible that you once knew it, but the lowering of your mind by the attack removed the memory from your brain....forever!
That's an interesting use for Mind points and gives your game a lot of scope for character development though roleplay - which I really like
it works rather nicely with the basic game and I'd love to institute something similar in my own games, but my problem is that once you've used the extra magic rules from the expansions Against the Ogre hoards and Wizards of Morcar, it becomes clear that the mind points are not a measure of intelligence but a measure of the Hero or Monster's strength of mind and thus his/her/it's ability to resist magical influences on their brain (in my houserules we eventually re-named the Mind stat to Will in an effort to avoid confusion)
~The ravings of a single mad Goblin is bad enough, but such a power-hungry, malice-filled creature as Mortis can never hope to be understood~
Ability at languages and inteligence are not necessarily the same thing. I know several biligual individuals who it constantly amazes me that they manage to dress themselves.
Perhaps you can reconcile this by treating the roll as an attempt to concentrate on understanding the language? Part of the charm of heroquest is its simplicity and I think the proposed idea ties in well with this.