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 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
Currently there is a bug with the Arabic and Hebrew translations that makes all the letters appear in the reverse order, probably due to these languages being right-to-left languages.
Ukrainian is called "Англійська (Україна)", which translates as "English (Ukraine). The correct translation is "Українська (Україна)".
Klingon has a similar situation, but since it's a language from off-world, it uses "tera'" which means "Terran," i.e. Earthling.
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 Klingon (Klingon language from Star Trek) and will soon be available in Quenya (Elvish language from Lord of the Rings).