I am an employee of a German company.

I am an employee of a German company.

If you are a new employee, you can call yourself a newcomer, or you can use fresh man or new fish. Personally, I think freshman means freshman. Don't associate freshness with man. In fact, greenhand also means a newcomer, but it only focuses on lack of experience, so it depends on the occasion.

Grass-roots personnel: junior professionals, also translated into front-line personnel, I think it is ok, grass-roots personnel generally work in the front line.

Middle-level employees: second-level employees or experienced employees (experienced)

Those who can be called high-level are generally leaders and managers. Don't translate them into employees, just executive managers, or use high to match translation.

Staff is a collective noun, which means all employees, that is, all employees. "An employee" can't be translated into "a staff", but it can be translated into a staff member, which means a member of the staff.

It's a little nerve-racking. I also learned a lot when I answered, because it took time to look up the information, which was worth it!