I notice that in the german localisation of Okular, some headings in the menu bar are missing, instead they are saying "Kein Text" ("no text"), for example the File section at the left (it should say Datei).
Also, my beloved entry Recently opened files is missing. There is not even an Open entry in the File menu.
On other computers with Ubuntu 18.04 that I've used, it works. It should look like this