Wilderlands: The Rise of Tharizdun
All languages in the Player’s Handbook are allowed.
In addition, sub-race languages may be taken. A sub-race language grants advantage on many social ability checks with native speakers, but disadvantage with speakers of other dialects.
For example, a character may choose “hill dwarf” as a language, granting advantage on many rolls with hill dwarves, but disadvantage on rolls with other types of dwarf. Or the character may chose “dwarvish” as a language (normal) and receive neither advantage or disadvantage. Multiple sub-race languages may be chosen.
For human languages, most humans speak common as a trade language. A character taking a human sub-race language gains advantage when speaking with that sub-race.
Learning a new language (or tool proficiency)
The character must find a teacher, spend 250 days and 250 gold (1 per day) learning the language. Roleplay may shorten the time and expense.