When I started this thread my main problems was that I was limited to the dictionary entries that the KT choose. For instance, when displaying an English book, the KT presented me with English-English definitions and English-French translations; when displaying a German book, the KT presented me with German-German definitions and German-English translations, and so on. There was no way to change the dictionary language (neither of the definition dictionaries nor of the translation dictionaries) or to get to the screen with the keyboard in order to modify the search string.
Today I checked the first seven dictionaries of the "Manage Dictionaries" list (even though I was convinced that the dictionaries were already at the device), pressed "save" and let the KT synchronize via Wifi. Actually, I did it in two steps, first only one dictionary and then the other six. To make it short, after two spontaneous reboots my KT behaves now more or less as expected. I can now even choose between different translation dictionaries, although I did not check them in the list.
It is however still impossible to open the edit screen (I mean the one with the keyboard) of the Japanese and the French definition dictionary. Switching the user language did not make any difference in this respect. As for the other dictionaries, I did not test all of them, but of those I did I am able to open the edit screen.
Thanks again to everybody for helpful comments.