Is it possible to get a certificate at the end of the course?
Duolingo has launched the Test Centre, but at the moment it only provides certifications in English.
If you want a certification in another language, you'd have to look elsewhere - there are exams you can take at various levels of proficiency in most languages. They usually cost a bit, though.
The certifications aren't hard to find: https://www.google.co.uk/search?q=German language certification.