We can distinguish between different situations
All foreigners (including non-EU citizens) and stateless persons who are not German citizens but have a German higher education entrance qualification are considered to be "Bildungsinländer." They are on an equal footing with German applicants. Please apply via the application portal.
Please apply via the online application portal. Your higher education entrance qualification will be converted into a German average grade by the Admissions Office for Foreign Applicants at the University of Tübingen. You will then take part in the normal internal school selection procedure, as you are on an equal footing with Germans.
If you are a German citizen but have not completed your Abitur in Germany, you will need a certificate from the Regierungspräsidium Stuttgart (Abt. 7 Schule und Bildung, Anerkennungsstelle, Postfach 10 36 42, 70031 Stuttgart, abteilung7; @rps.bwl.dehttps://rp.baden-wuerttemberg.de/rps/Abt7/Seiten/certificate.aspx) or from a Ministry of Education of another Federal State of Germany, from which it is evident that the proof of educational achievement is equivalent to a German school leaving certificate. This certificate must contain the date of your higher education entrance qualification and a conversion of your grade into the German grading system.
International applicants who have not acquired their study qualification at a German-speaking institution must prove German language skills that enable them to study at a university ("Sprachliche Studierfähigkeit"). Further information can be found here.
Exceptions are the degree programs M.Sc. in International Economics, M.Sc. in Economics and Finance, M.Sc. in Economics, M.Sc. in European Economics and M.Sc. in Management & Economics, in which you can prove your knowledge of English instead.