a. I graduated from the department of Political Science at x University, in yland. As far as I know my university's equivalence with the German universities is recognized by the German higher education system. However, because of the differences between the both higher education systems , do I have to take additional courses in order to start the Master programme? or do I have to take "Gleichwertigkeitsprüfung"?
Concerning the recognition of your degree from your university, we - at the Institute of Political Science - cannot give you any information, unfortunately. You would have to check with the University Advisory Service ("Akademisches Beratungszentrum"), which has a department which is responsible for advising and admitting international students. I will give you the address. From them you will also get application forms, information on deadlines, language prerequisites, etc.:
Wilhelmstr. 11, 2. OG.
+49 7071 29-72555
b. My German is not sufficient to study in German. Does the university provide preparatory German courses before beginning to the degree?
If you are interested in preparatory language courses: they are offered by the department of "Internationale Sprachprogramme": http://www.uni-tuebingen.de/en/facilities/verwaltung-dezernate/division-iii-international-affairs/german-as-a-foreign-language-and-intercultural-programmes/learn-german.html
c. Would it be usefull to add letters of recommendation to the application?
To promote the recognition of your degree as an equivalent degree it would make sens to add recommendations concerning the performance of your studies. The examination will be executed by the University Advisory Service in cooperation with the Institute of Political Science.
d. Should the recommendation letters be sent by the professor or shall I add them to the application documents?
Please add them to the application documents.
e. Must the recommendations be written in German or is English sufficient?
They can be written in English.