Today I became aware that it is again possible to add custom dictionaries to the installed dictionaries (and not only to replace existing Kobo dictionaries of the same name). I tried dicthtml-fr-fr.zip and dicthtml-de-de.zip. After restarting the reader, both were in the list and selectable, but not searchable. See an example here
. After adding for each new dictionary a line in the Dictionary table of the database, both dictionary became searchable.
This seems however not to work with dictionaries that translate from (and possibly to) Japanese.
Be aware, that the database is not much forgiving. It happened to me that I wrote de-de instead of -de-de into the first column. The reader refused the whole database and asked me to initialize the reader via wifi or usb. The same happened when I tried to write English - Farsi in the name field instead of Deutsch - Deutsch.