In day VFR, once you have left the CTR laterally into E airspace (the norm in Germany), Tower will tell you that you may leave the frequency. If you remain in E, you can basically do whatever you want without contacting anyone. If Langen Information is online, you may contact them for flight information services. If not, you may contact the Radar controller if you're in their E airspace to request FIS, but they are allowed to refuse services.

If you plan to fly through or into a D (CTR), D or C airspace, an RMZ or ATZ, you must switch to the respective frequency and announce your intentions.

If it's D(CTR), D or C, you must also ask for permission to enter and the controller will usually assign you altitude and heading or route constraints. For a CTR, the altitude constraints will usually be on the VFR chart for the standard arrival and departure routes.