The choice of languages is a feature that was introduced in weekly snapshot 11w49a and officially released in Minecraft 1.1. The language menu is accessible via a new button in the main menu to the left of the options button, and from the options menu.
Unknown (probably because Austrian German mostly equals standard German)
Español (La)
Latino-American Spanish
South America
11w49a (1.1.0)
12w01a (1.1.0)
Replaced by Argentinian, Venezuelan and Uruguayan Spanish translations.
Magyar
Hungarian
Hungary
11w49a (1.1.0)
12w01a (1.1.0)
Unknown
Malti
Maltese
Malta
11w49a (1.1.0)
12w01a (1.1.0)
Unknown (removed from the Translation Project page as well)
Bugs & Inaccuracy
Bulgarian is doubled, and both are wrong — as български (means Bulgarian, but written from the small letter instead of the capital one) and as Английски (written from the capital letter, but means English). The Ukrainian language has a similar problem - a list he called "Англійська" (English). The same applies to the Basque, Greek, Icelandic and Romanian languages.
In some languages, the text is too long and overfills the button it is written on.
Splash texts are not translated and therefore will show up in English.
In Canadian English, the "Benchmarking" achievement description is bugged. Also, Gray and Light Gray Wool spell the word "Grey", but the dye spells it "Gray".
Power and punch enchantments on bows have the name of the similar sword enchantment when using Canadian English
Many Turkish translations contain obscenities, which were contributed by trolls.
There are some critical mistakes in some of the Translations, like Hebrew.
The UK English and Canadian English use the other languages code.
I.E: Name = Canadian English, Language code = en_GB and vice versa.
In Pirate, the "Industrious now, aren't we?" achivement has a w on the end.
Trivia
Minecraft is available in three fictive languages : Klingon (Klingon language from Star Trek), Pirate (Internet language) and Quenya (Elvish language from Lord of the Rings).
In Pirate speak, Gold Boots are called "Buttershoes", a reference to Seananners, a well known YouTube personality that ran a Minecraft series.