1)Common Tongue
2)Barbarian
3)Combine Tongue
4)Dark Speech
5)Dark Elvish
6)Dragon
7)Dwarvish
8)Elder Elvish
9)Elder Dragon
10)Elder Tier'Dal
11)Elvish
12)Erudian
13)Faerie
14)Froglok
15)Gnoll
16)Gnomish
17)Goblin
18)Halfling
19)Lizardman
20)Ogre
21)Old Erudian
22)Orcish
23)Thieves Cant
24)Troll
25)Vah Shir
26)Kobold
27) Akheva (Rumored) I have yet to find this language or anyone that can teach it... although I've only been looking for about 15 minutes.
28) An Unknown tongue - I have yet to verify this actually exists but on several threads I find people that claim to have learned and maxed this mythical secret language.