When writing about work experience in CV, should I mention if a job was part-time? Also, should the employment certificate clearly mention the work hours of the job?
Your Arbeitszeugnis can mention it. You as an employee have a right to have your weekly time in there if you want. There have been no court rulings for any case where an employer put it in and an employee did not want it. As somebody reading it later, I won't mind if it's left out, as long as it's mentioned somewhere.
There is no clear cut rule whether you should mention that it was not a full time job on your CV. Personally, I'd say if it's 30h/week or more it's okay to leave it of and just mention it in passing in the interview, if it were 20h/week or less, I'd feel a little cheated as a reader if it wasn't mentioned explicitly. After all a 5 year job at 20 hours a week nets about 2.5 years of experience compared to another person who worked full time.