Actually libertarian is pretty close to the right translation for the FDP since the word liberal has been redefined in English to mean the political left (though not socialist, which in the US has also been redefined to Marxist-Leninist). Libertarianism, in American English, is closer to a classical laissez faire liberalism which the FDP proclaims.
(Note: I'm an American that's been living in Germany for the last 7.5 years.)
OK. I thought libertarian meant more or less Anarcho-Capitalist, which the FDP aren't. I guess the FDP is pretty close to the Economist on most issues.
(Anyway, every party in Germany is a socialdemocratic party in some sense.)
The FDP (free democratic party) had its best showing at a federal election ever, with around 15% of the vote. (So did the socialist party with around 12% or so.)