Hi websource,
Thanks for getting in touch. The .pot files for The Events Calendar and Events Calendar PRO had a lot of changes in version 3.5, which is why some of the strings are now showing in English. Lucky for you, one of our other German users just updated the files so that those strings are translated. You can download the new German files from here. Drop the two for The Events Calendar into that /lang file (replacing the old ones) and do the same for the Events Calendar PRO files. That should get those strings translated properly.
Let me know if you need any help.
Best,
Leah