The Polytechnic Diploma is not accepted as a German "university entrance qualification". A direct admission to German universities is not possible.
However, you qualify for subject-restricted admission to universities of applied sciences. You can gain this admission by visiting the so called "Studienkolleg" (http://studienkollegs.de/home.html) at universities of applied sciences in the subject area corresponding to the key area of your diploma. The Studienkolleg is a one-year termed foundation course. You have to pass an entrance examination to participate in the foundation course. Good German language skills are a prerequisite (B1-B2 level of the European Framework of Reference for Languages).
At the conclusion of the foundation course, you will be tested in several subjects which are relevant to the subject of study you intend to pursue. This test is called the "university entrance assessment examination" ("Feststellungsprüfung"). One part of the examination is a language test.
Most universities do not charge a tuition fee for foundation courses. However, as a course participant, you are required to pay the semester contribution of the university.
There are also a few private foundation courses that require less German language skills or even sometimes no knowledge of German at all. The private foundation courses charge higher tuition fees.
Exception: Polytechnic Diploma 'with Merit' or acceptance letter from NUS or NTU
With a Polytechnic Diploma "with Merit" or with a Polytechnic Diploma with a certificate (offer letter) of the National University of Singapore (NUS) or the Nanyang Technological University (NTU) you qualify for a direct subject-restricted admission for all higher education institutions. This allows you to apply directly to a German higher education institution for academic studies in the subject area corresponding to the subject area of your diploma.
NOTE: As an additional prerequisite in all cases you are required to provide your O-Level results. Among the 5 examinations subjects 2 need to be languages and one a mathematical or natural science subject.