All humans, as well as those semi-humans and non-humans in close contact with people, speak the “common tongue”. This language is spoken by all states in the central campaign area, but your referee may well have areas in which the common tongue is different from that which your character speaks.
In addition to the common tongue, all intelligent creatures able to converse in speech use special languages particular to their alignment. These alignment languages are: Chaotic Evil, Chaotic Good, Chaotic Neutral, Lawful Evil, Lawful Good, Lawful Neutral, Neutral Evil, Neutral Good, and Neutrality. The alignment of the character will dictate which language he or she speaks, for only one alignment dialect can be used by a character (cf. The Assassin). If a character changes alignment, the previously known language is no longer able to be spoken by him or her.
In addition to the above, druids have their own special tongue, and thieves have their secret speech, the Thieves’ Cant. Therefore, a character will speak at least two languages—common and alignment. He or she might also be able to converse in the special patois of druids or thieves.
Semi-human and non-human characters are able to speak racial tongues as well. (See Character Races). In most campaigns, it is likely that open alignment speech will be frowned upon as a serious breach of social etiquette.
A character can learn additional languages. Even the rather slow (80 I.Q.) can learn one additional language. However, his vocabulary, usage, and ability to translate must, perforce, be limited. The very bright can learn five, six, or even seven. (For details of the number of tongues which can be learned see Intelligence).
The character must find a person (human, semi-human, or non-human) or talking creature who speaks the language which is to be learned. The character must then be in close proximity to his or her instructor for up to one year, and prior to that period the language can be learned one month early for each point of Intelligence above 12. Thus, a character with 13 Intelligence learns the new language in 11 months, 14 Intelligence learns in 10 months, all the way to 18 intelligence which requires but six months to learn. Only one new language can be studied at any given time.
Which languages are knowable in a campaign is strictly up to the DM who runs it. The following list gives some of the languages typically used:
All of the above may not normally be available due to campaign circumstances, and languages not listed might be common as determined by the Dungeon Master.
Note that if, for example, you capture a centaur or talking dragon of some sort, it is possible to learn the appropriate creature tongue.