Hi Nadiuska,
The most important language pre-requisite for Skyguide is English, Level 1 Oxford or Cambridge written and spoken. You will also have to pass a Eurocontrol proficiency in English language test as well which is administered in all EU states.
Yes, you will coordinate with other sectors in German, but that will surprisingly come easily once you immerse yourself in it, and it is a more technical and job specific language.
On the plus side and from personal experience, Skyguide offers the best ATC training programme that I have experienced working in 6 countries, if anybody can make you into a sucessful controller then the Skyguide instructors can
Good luck with your application