For an introduction in English see Chapter 9 of book below:
"The Federal Republic of Germany is the depositary of more than 20 multilateral treaties. The wording of these treaties, status lists as well as declarations by contracting parties have been made available on [Federal Foreign Office] website since September 2011 and can now be found also in English."
Before an international treaty can take effect, a federal legislative procedure must be completed. See Article 59 II Grundgesetze
The texts of international treaties (& the related federal law) are officially published in the Federal Law Gazette Bundesgesetzblatt Teil II (BGBl.II in citations) The annual Fundstellenverzeichnis B indexes the Bundesgesetzblatt II chronologically, by name of participant state and by subject.
The LawBod has the print volumes at German 010 1949
The easiest way to find the text of bilateral investment treaties is often to use UNCTAD free Investment Instruments Online database - see link below.
Alternative sources for multilateral treaties (which may also supply an authentic text in English) are the League of Nations Treaty Series (LNTS) and the United Nations Treaty Series (UNTS), shelved Internat 10. OU members can access both electronically via HeinOnline, however searching by name of participant country does not always work. The free, online UN Treaty Collection site (see second link below) may be better for participant searches.
The LawBod's copy of Sartorius : Bd. 2. Europa-Recht und andere internationale Verträge [later] Internationale Verträge-Europarecht at
Internat 500 S251 was last updated in 1993.