How to translate German "Wissenschaftszeitvertragsgesetz"?
I wouldn’t translate the law’s name as I do not see how it helps potential applicants. Just put yourself in their position and think about what benefit it grants you:


Any reasonable resource elaborating the content of the law in detail will mention the German name, even if written in English.
The translation itself will only give the applicant a very rough idea what it is about. The more important aspect what relevant consequences the law has.
To be actually affected by the law, the applicant has to have a history of employment in German academia and thus not be scared by long German words anymore.


If I would have to write about this in a job ad, I would write something like:


  Applicants who were previously employed in a similar position in Germany may not be eligible due to the restrictions imposed by German law (Wissenschaftszeitvertragsgesetz).


Note that I have rarely ever seen this law mentioned on job ads to which it applies and never on a PhD position – which makes sense because the law would only matter if the candidate already has been employed on a PhD position in Germany for more than three years.

Finally, if you can, I would let your HR department worry about this. They should have some legal boilerplate available.