I commonly see people ask about language and literacy in the Ninth World. What languages does a beginning player character speak? Can PCs read and write?
There are two sections in the Numenera corebook that deal with language: The Language and Literacy sections on pages 132-133 and the Language section on page 339.
The Truth is the only language widely spoken in the Ninth World.
The Truth is the predominant language in the Steadfast, where it is spoken by about 80 percent of the people; in cities, that number is closer to 100 percent. In the Beyond, about 60 percent of the people speak the Truth as their primary language, but many isolated villages have their own tongue (Numenera, page 133).
Characters start out knowing the Truth and maybe one other local language/dialect that fits their background. As the game progresses, they can buy additional languages as a short-term benefit for 2 XP each (it's short term because, other than the Truth, each language is only spoken in a very small area, and it's likely the PCs won't be there long).
Characters should begin the game knowing the language(s) that make the most sense for them. For the vast majority, that will be the Truth, and maybe a bit of some local dialect or unique language. As the game progresses, characters can learn new languages by spending XP [2 XP] to gain short-term benefits. More than likely, learning a new language will have only a limited or short-term benefit because most languages other than the Truth aren’t widely used (Numenera, page 339).
On page 132 of the corebook it states that in the Steadfast about 50% of people are literate, and in the cities about 70%, although most people can recognize a few words in the Truth. I think it's safe to assume that PCs fall into the percentage (however small) of literate people unless a player specifically wants to play an illiterate character.
Not only does every settlement beyond the Steadfast likely speak its own language, but many of the creatures, abhumans and visitants of the Ninth World have their own languages as well.
Although you as the GM can ignore language in your games, I highly recommend that you don't do that. Communicating with entities that don't speak their language is a unique challenge to PCs that will really add a new degree of depth to your games and roleplaying.