From the game's encyclopedia
A Language is the Cultural Pillar that determines what language the general populace of a Culture speaks. Cultures which speak the same language gain a permanent bonus to their Cultural Acceptance Baseline.
A Character can know one or more Languages. Knowing a language cuts the Cultural Acceptance Opinion penalty in half towards Characters and Counties that speak it. It also discourages Independence and Claimant Factions.
Characters can learn additional languages by using the Learn Language Scheme. Characters can comfortably learn languages up to their Foreign Language Limit.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Language is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Language is the Cultural Pillar that determines what language the general populace of a Culture speaks." It covers Foreign Language Limit and Court Language. The game also links it as Languages. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.