The choice of languages is a feature that was introduced in weekly snapshot 11w49a and will be added in Minecraft 1.1.0. The language menu is accessable via a new button in the main menu to the left of the options button, and from the options menu.
Available Languages
The languages available are, in order as they appear in the menu :
Bulgarian
Czech
Welsh
Northern dialect
Southern dialect
Manx
Danish
German
Austrian dialect
German dialect
English
Canadian dialect
British dialect
US dialect
Spanish
Argentinian dialect
Spanish dialect
Latino-American dialect
Estonian
Finnish
French
Canadian dialect
French dialect
Hebrew
Croatian
Hungarian
Icelandic
Italian
Korean
Japanese
Lithuanian
Maltese
Norwegian
Bokmål dialect
Nynorsk dialect
Dutch
Polish
Portugese
Brasilian dialect
Portuguese dialect
Russian
Slovenian
Swedish
Turkish
Chinese
Trivia
There are 28 languages (36 if you count the different dialects for each language).