Currently, Skype Translator supports English to and from Chinese (Mandarin), Italian and Spanish. Microsoft is encouraging testers to use it with these languages.
Microsoft's ultimate plan is to support 45 more languages with Skype Translator and to make the service available across a variety of devices running different operating systems.
Update: A Microsoft spokesperson provided clarification about what they're testing as part of the open preview:
"The Skype Translator preview is only available for speech translation in the following languages (to and from English): Spanish, Italian and Chinese (Mandarin). We also have 50 IM languages available. The list of languages that you have noticed on the sign-up page (which includes German and French) are languages in which the Skype Translator client is available - i.e. localization of the app for translation of the languages that are currently available."