Hello Aseman,
as you said, you can usually only study in Germany if you have mastered the German language to a certain level. Accepted forms of proof of language proficiency are:
DSH - Deutsche Sprachprüfung für den Hochschulzugang ausländischer Studienbewerber (German Language University Entrance Examination for International Applicants)
TestDaF - Test Deutsch als Fremdsprache (Test of German as a Foreign Language)
GDS - Großes Deutsches Sprachdiplom des Goethe-Instituts (Goethe Institut German Language Diploma)
DSD - Sprachdiplom der Kultusministerkonferenz Stufe II (German Language Diploma of the Standing Conference of the Ministers of Education and Cultural Affairs, Level II)
Information source with clickable links: ****://bit.ly/1VFYcL6
A detailed example provided by the Freie Universität (FU) Berlin: ****://bit.ly/1VFYhyt
Kind Regards, Jan