Added, thanks.
As far as I understand, Kobo devices will load dictionaries only if the language (monolingual) or language-language (bilingual) correspond to one of the official language/language-language. In your case, you had to fake e.g. "English-Latin" as "English-Italian" because only the latter is officially available from Kobo.
About why Kobo does not support custom dictionaries: I have no decisive clues, as I do not work for them, however I guess it is just a matter of business opportunity: they decided that implementing the feature will not bring in enough cash to cover the associated costs (developing the code, integrating it, documentation and support), and they simply have not done it.
From time to time I bug them on Twitter, linking to the first post of this thread and asking at least for official documentation of the dictionary format, and I have never heard back. E.g.:
https://twitter.com/acutebit/status/555299571832979456