In many RPGs language is a last consideration, if at all. However, the use of different languages in a game can dramatically influence the flavour of the world. Having barriers to easy communication can add new challenges for the players, can create ways of including or excluding players, and can open up secrets to the players.

Ordering food from the tavern or inn when you do not speak the tongue can give some comic role playing opportunities. Things like bartering the price on some new horses also points to cultural differences. For example, in the first Hobbit movie the Dwarves of Thorin’s company dislike the Elven food of Rivendale, which shows up the differences between to two cultures and how they interact with the wider world.

Another situation is when one or two players can talk among themselves, but exclude others. For example, again in the Hobbit, when Gandalf and Elrond speak in Elvish they excluded the Dwarves who react negatively. This again shows up cultural differences.

The final point about languages is when NPCs, or villains assume that the PCs do not understand them. For example in Monument Men, a US soldier is standing close by the captured German soldier and over hears their conversation, which leads to the gaining of additional information.

Overall, the use of multiple languages can add richness to your worlds, but it must be done with care, because it can create barriers that can divide the group and distract from what you want to achieve in the campaign. I’d always make sure that every group does have a common tongue that they share, like Common in D&D. It’s best used sparingly to enhance the drama, not to restrict it.