Thank you Susan, that gives some kind of direction.
I can't get it to work though. Here is a case:
I have a Latin dictionary bought here: http://www.amazon.de/Electronic-Lati...=pd_sim_kinc_1
The Amazon page has even a set of directions to set up the dictionary as a "primary" one. It is obsolete, however, because K4 has seemingly no "primary" dictionary, but a set of defaults for various languages (probably a predefined small choice). In any case, there is no "primary dictionary" setting as described.
So, the Latin dictionary is on the device, not configured in any way, but, as you say, (pre)formatted to be usable as a default.
I also have Virgil's Aeneid as a test case, downloaded from Project Gutenberg and converted either on that site, or by Calibre. Calibre shows "Language: latin" in metadata.
And... no pop-ups.