TL;DR: Ask them what exactly they need and when. If you guess what they want, there is a good chance you are wrong and waste everybody’s time.
I am here writing as a German who has applied some jobs with similar and weird requirements and has some insights into how those happen. Also I am assuming that you are doing some specialised job, usually requiring an academic education, because otherwise you are unlikely to be in a such a scenario.
Such requirements usually originate from some interplay between clueless bureaucrats or HR people writing the job ads, overly rigid rules, and bad communication but do not come from the people actually making the hiring decisions. What is written on the job ad or in any canned message is almost certainly not reflecting what they really need or want.
There are two categories of reasons for asking for these certificates:
They need some proof that you actually worked in the respective positions or performed certain tasks. For example this could be due to some legal requirements for a job, to determine your salary class in public service, or to check that you are not blatantly lying on your CV. In this case, other proofs of work will probably be satisfactory, down to a work contract and some salary summaries.
They want to gauge your qualities and work morale. For this purpose, a formal Arbeitszeugnis is arguably close to useless, in particular in specialised positions. Typical reference letters should suffice and actually be preferable.
To know the actual reason and specific requirements, you have to ask them. I would strongly recommend this since depending on the reason, possible substitutions can widely differ. If you just make an educated guess, you may waste several people’s time and even fail at this hurdle.
Note that you cannot guess the reason from the stage at which they require the certificates. For example, even if the certificates are only needed for the hiring process, sometimes they are prematurely required at the application stage to speed up things, when they decide to hire you. In this case, whoever imposed this requirement has probably not considered that it drastically und unnecessarily increases the application effort for people like you. Therefore it might also be worth inquiring at what stage they really need the certificates.