language_is_shown_trigger

language_is_shown_trigger = {
	scope:character = {
		OR = {
			AND = {
				exists = var:legendary_culture
				var:legendary_culture = { has_cultural_pillar = $LANGUAGE$ }
			}
			AND = {
				# The target language needs to fulfill ALL of the following:
				any_sub_realm_county = { # Language needs to be well-represented within your realm
					percent >= 0.1
					culture = { has_cultural_pillar = $LANGUAGE$ }
				}
				any_sub_realm_county = { # Language needs to border your own culture
					culture = { has_cultural_pillar = $LANGUAGE$ }
					any_neighboring_county = {
						culture = { this = scope:character.culture }
					}
				}
				any_sub_realm_county = { # You need a minimum acceptance with any culture speaking the language
					culture = {
						has_cultural_pillar = $LANGUAGE$
						cultural_acceptance = {
							target = scope:character.culture
							value >= 25
						}
					}
				}
			}
		}
	}
}

As written in game/common/scripted_triggers/00_cultural_triggers.txt.