I graduated and got an M.Sc. degree from a German university. I got a grade of 1.0 after my master-thesis defense. In this case, can I use "summa cum laude" on my CV?
I would think that would be a dangerous move. List what you have the way it was formally granted. If you think it necessary provide a reference to an official definition so that those who don't understand its significance can learn to do so.
But it isn't really the same thing. Summa Cum Laude is usually (often at least) in the US based on grades for coursework.
If you are suspected of trying to mislead, it will be bad for you. Tell it like it is.
No. (Unless your Prüfungsordnung says that so explicitly.)
The longer answer: The grading scheme in a German Masters Program is usually handled by the Prüfungsordnungen (could be more than one - an general one and a specific one...). Of course, there is a scheme that is used by most universities in Germany (1 - very good, 2 - good ... and so on) but actually, every Prüfungsordnung handles this issue. It may be that your Prüfungsordnung has a paragraph where it says that there a "mit Auszeichnung" (which should be translated as "with distinction") but if there is no such thing, than you should not list anything. The term "summa cum laude" is, as far as I know, only used for the doctoral thesis in Germany. Where I am, the Masterurkunde comes with a Diploma Supplement in English which explains the grading system (and, for that matter, also the German system of education), so if you have a similar thing, you could also look there for a proper translation of your grade.
When I applied to jobs outside of Germany, I listed all grades as given and included a footnote explaining the German grading system:
Grades in Germany are usually awarded on a scale from 1 (best) to 6 (worst).
with a link to the Wikipedia article Academic Grading in Germany. Any implicit conversion could be considered dishonest or would at least be confusing to those familiar with the respective grading system.
A particular problem with listing summa cum laude or similar is this: Some degrees in Germany can be awarded with an additional honours attribute. For example, for one of my degrees, when you got a grade of 1.0 for all modules (coursework and thesis), your thesis would be graded by an external examiner. If this examiner also awarded a grade of 1.0, your degree was equipped with the attribute mit Auszeichnung (with honours). Otherwise, it would usually still be a 1.0. Thus stating your degree as summa cum laude or similar could be understood that you got this kind of a degree – even if your department does not have the corresponding process.
No. Schools award honors. One does not give honors to oneself. If the school has not informed you of honors, you can't claim them. You can include your good score on your CV if you want.
If you did not receive mit Auszeichnung recognition for your thesis and general coursework then there is absolutely no validity to a claim of summa cum laude. A 1,0 grade is necessary but not sufficient.
Moreover, you’d need to have some guidance from the university about how the degree can be described. Most likely, the regulations will say that if you did receive such recognition, you would declare it in English as being “with honors,” which is a legitimate translation.