Patrick S, the False Machine, made me think about languages in RPGs. This is one of those areas, I feel (like underwater adventuring and flying combat) which is still crying out for somebody in the OSRchipelago to revolutionise with something really useful, fresh and compelling. I will by no means attempt to do that here. But here are some thoughts about how to make languages interesting and meaningful.
1. Areal Common Tongues
Different jargons and lingua francas function in different parts of the map. So you can speak one sort of 'common' to all the monsters in hexes AA, AB, and AC, but have to use a different kind (or a translator) in hexes BA, BB, and BC. Adds a hurdle to exploration - the good kind of hurdle, which requires more engagement with the game world.
2. Translators as Resources
Real-life explorers, I once read, often used young girls as translators, because being young and female is generally correlated with excellent language-learning ability. The PCs having to keep safe a frail adolescent on their adventures could add a lot of nuance to proceedings. But equally, you could just say that the common tongue is really difficult and requires so much training (or special equipment like whistles, flutes, drums, and so on) that it requires an expensive specialist. The PCs have to pay money to hire one, and have to stop the otherwise useless NPC from being killed into the bargain.
3. The Common Tongue is Not Just a 'Tongue'
Maybe the common tongue incorporates pheromones, or the use of coloured paint, or some other expensive or rare chemical (of, if you want to go more exotic and strange, maybe some 'common' phonemes can only be produced by a species of insect or bird that you have to carry around with you and encourage to squeak at certain moments). It's expensive to pay for all this stuff - or it is a rare resource the PCs have to search for, and husband. (Do we really need to parlay with these orcs? No, we can't waste the pheromones!)
4. The Common Tongue is Dangerous
Speaking common is not to be trifled with, because talking to a dragon, orc, elf, hook horror or whatever is to have one's consciousness moulded, however fleetingly, by alien concepts and thought-patterns that, because one understands the words being used, one is forced to entertain. The effort can result in madness or corruption, and whenever one speaks common there is a small risk of either of those or both occurring.
5. The Common Tongue Has to be Performed
The common tongue is a crude communication tool which you actually have to deploy at the table. Maybe it's an SOS style system of taps and pauses. Maybe it is comprised of words, but there is a strictly limited list of, say, 51 or 101 that you can use. Maybe it is made up of gestures. Much hilarity ensues.