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 had no difference with the standard German translation)
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
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).
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.
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).
An early bug that came with the implementation of languages made right-to-left languages (at this time Arabic and Hebrew) be written left-to-right, causing it to be unreadable. This was fixed in version 1.1.