arspr,
Most of your thoughts are outside of my scope, but I would like to comment on your subsection entitled
"Additional translation values for Tags and Tag-like custom columns".
Tags are usually downloaded metadata. Amazon, for example, bases its categories (i.e., Tags) on BISAC Subject Headings and Codes. BISAC is the acronym for the Book Industry Study Group. It is based (unsurprisingly) in New York, where the publishing industry is clustered. Everything BISAC publishes for use by real Publishers is in English, as far as I can discern. Their current list can be found at:
https://www.bisg.org/complete-bisac-...s-2013-edition .
When you see a Tag in Calibre named "Thriller", it likely was assigned to a particular ebook by its publisher. The publisher was given the BISAC codes for a particular ebook by its author, because the author presumably knows best about what their book is about.
For example, the BISAC Subject Heading FIC030000 has the English meaning of "FICTION / Thrillers / Suspense".
The entire list of BISAC Subject Codes can be downloaded from BISAC from
https://www.bisg.org/publications/bi...s-2013-edition for $295.
If someone were to translate the BISAC Subject Codes into a particular language, and if those translations were imported into a special static reference table in Calibre, then the standard Calibre API could be used to acquire the translations from that (currently non-existent) table for the purpose of translating Tags that had a match.
However, there is an immediately available alternative option. My plugin 'Derive Genres' is multilingual, and can be used to populate the custom column Genre (#genre) using Boolean Tag Rules. The examples that I provide with that plugin in order to jump-start its users were inspired by Amazon's categories (Tags), which themselves were derived from BISAC Subject Codes. The few test data examples shown in the attached .jpg file are in Spanish, Catalan, Portuguese and German. The Boolean Operators (AND, OR, and NOT) are supported only in English, Spanish, French, German and Hindi because I had to hardcode their translations to English for use in the Boolean Equations, but everything else is fully user-defined since the rules themselves (maintained in a .csv file) must be encoded in Unicode (UTF8). So, your English Tags can be turned into Spanish Genres. Or, you can mix and match and have Fiction genres in Spanish and Nonfiction in German, with the exception of Romance which could be in French or Catalan. My point is that it is highly flexible.
If you have any interest, I refer you to its 19-page instructions/user-guide available within the plugin for more information.
Best of luck.