In the french academic system, master's programs last for 2 years and one can start from the second year (if he/she fulfills the academic requirements, such as having already studied the first year or has already another master's degree in the same specialization). From what I saw in some german universities websites, master's degrees in germany last 2 years, but they don't precise whether one has the possibility to apply for an entry from the second year (if for example one has already a master's degree in the same field). Is it possible to join from the second year?
The best way to answer your question is to explain that it's a two-step process:
First, you need to be admitted to the master's program. If you're coming in as an international student, this can be in and of itself very difficult, particularly if you are applying to what is known as a "consecutive" master's degree program, for which the usual prerequisite is a bachelor's degree in the same field from a German university. If you are coming from a different field, or from outside Germany, you need to have the university recognize and accept your credentials as substantially equivalent to the bachelor's degree in question.
After you have gained admission, you can then proceed to establish that courses you have taken are equivalent to the master's courses offered in the degree program of interest. Normally this has to be approved by the instructor of each course you're interested in getting "transfer credit" for.
Thus, you won't be admitted into the "second year" or "semester n" of a given master's program, but you can get the equivalent credit, which will thus shorten your overall time-to-degree accordingly.