In general in Germany, the only requirement to enroll in a Masters program at a public university is a finished Bachelor. In most cases no-one cares how you got your Bachelors degree and how long that took.
HOWEVER: popular studies have an NC (numerus clausus), that changes every year depending on the popularity. The NC is the limiting grade with which you are able to be admitted. The more people apply for a certain program, the higher the NC. For the NC, only the overall grade of your Bachelor degree counts, not how much time you took to obtain that grade.
There are nevertheless quite a lot of programs that don't have an NC, meaning there are in general not more applicants than spots. This means that basically anybody who fulfills the basic requirements (in case of a Masters program: finished Bachelor in relevant subject, visa requirements, language requirements, ...) will get a spot in that program. I bet there are quite a lote of math masters (and related subjects) without NC. So even if your Bachelor grade is not the best, you might be able to find a spot.
The comments of SirHawrk about the language issues are important, though - although many Germans speak English, a lot of university life (and administration) is taking place in German.