This is confusing because several of these terms can have more than one meaning. In English, in my experience, ...
... a line basically means one horizontal mark on the page (= Linie), the thing you can draw with a ruler, the thing you study in geometry class.
However, it can also mean one part (= Stimme) in a larger score: e.g., the dynamics are marked only above the soprano line, but not in all the voice parts.
A staff, plural staffs or staves, is normally a group of five lines in the first sense (e.g., in the treble clef, E G B D F).
However, it can also be a group of two such staves, that is, a piano staff (which consists of a treble staff and a bass staff, usually joined by a curly bracket).
A system is normally a group of several staffs: for example, a piano staff plus one staff each for SATB parts; or an entire huge system with one staff for each orchestral part. It's usually joined by a straight bracket.
(OT: Is 'accolade' really the technical term for these brackets? If so, that might be an interesting new entry.)
In a music notation or score-writing program such as Encore, you have the option of choosing how many staffs per system and how many systems per page.
Music with only one or two parts, like a piano score with a vocal line, can have as many as six or eight systems per page. But in a larger choral or orchestral score, unless it's printed in microscopic type and on enormous sheets of paper, it would be very unusual to have more than two or three systems per page, because each system has several lines/staffs and takes up a lot of space.
If the German word Notensystem really means primarily a staff (= one part / eine Stimme) and only secondarily a system (= many parts / mehrere Stimmen), that might indeed be useful to note, since it would be a false friend. Maybe it's just that I'm confused, but I'm not sure that's entirely been established from the discussion so far.