Serbian term (edited): непријављивање запослених

undeclared work means any paid activities that are lawful as regards their nature, but not declared in conformity with national law and practice. In any case, this definition must not be more restrictive than the legislation in force in each Member State;

https://www.lawinsider.com/dictionary/undeclared-work

Nota Bene: "any paid activities that are lawful as regards their nature" - not quite the same as "undocumented" workers, where the "lack of documents" is not about "forgetting" to notify the authorities, but about the employed workers not having the right "documents" i.e. "not having the right to work".

1. What is an "undocumented worker" or "undocumented immigrant?"
In the United States, an undocumented worker or undocumented immigrant is a foreign-born person who is not a permanent resident and is not a U.S. citizen. "Undocumented immigrant" may refer to a person whose immigration status is not resolved. Due to the unresolved status, the worker does not have permission to work in the United States. For information on types of immigrant status visit the U.S. Citizenship and Immigration Services website.

https://www.workplacefairness.org/undocumented-workers

This term "непријављивање запослених" is about employers abstaining from declaring their workers to the relevant authorities. In Serbia, the vast majority of the "undeclared" would be perfectly entitled to work in Serbia - they would be definitely "documented".