It looks like it was originally released in English, according to this post on their forums:
The English language version of X³: Terran Conflict will be available from retail outlets on October 10th, and you can pre-order from the Egosoft Online Shop NOW. X³: Terran Conflict will also be available for pre-load from Steam starting one week before release.
However, comments on another post suggest that perhaps both languages were available at launch:
Yes, during the installing process, your computer ask you, what languages you would like to install
and this post seems to support that theory:
Wenn die deutsche Sprachdateien installiert wurden, kannst du die Spiel-Sprache, wie im FAQ-Artikel Ich habe mehrere Sprachen installiert. Wie ändere ich sie? beschrieben, auf Deutsch umstellen.
(roughly: If the German language files are installed, you can switch the language in the game to German, as described in the FAQ article "I have multiple languages installed. How do I change it?")
Note that neither post specifically says German was available at launch, but I was not able to find evidence that the German-language version was released later.