I'm trying to create a slightly modified translator for biblatex exports (based on biblatex.js). Despite my lack of javascript knowledge :/ I've been able to figure most of it out based on other discussions here on the forum. But here are a few things I'm still having trouble with:

Is there a way to replace “typographic quotes” with "straight quotes" in any of the fields, or, even better, the proper ``latex quotes''?

Then, for entries that don't have a subtitle, is there a way to make it duplicate the (entire) title to the shorttitle field?

Note that most of this can also be handled inside your latex document using biblatex macros, but if you insist:

https://retorque.re/zotero-better-bibtex/scripting/ :

if (Translator.BetterBibLaTeX && !this.has.shorttitle) this.add({ name: 'shorttitle', value: item.title })

I know that biblatex includes day and moth in the "date" field, however, I don't use those, and try to keep every entry minimal. Is there a way to strip the -MM-DD information from the date?

if (Translator.BetterBibLaTeX && item.date) {
  const date = Zotero.BetterBibTeX.parseDate(item.date)
  if (date.type === 'date') this.add({ name: 'date', value: date.year })
}

Lastly, for book entrytypes is there a way to make it duplicate the title to booktitle (for traditional bibtex crossrefs)?

if (Translator.BetterBibLaTeX && item.itemType === 'book') this.add({ name: 'shorttitle', booktitle: item.title })

EDIT: And one more: I'm also trying to get rid of the Googlebooks ID, which gets written into the notes field. Any way to omit that?

BBT will probably have stripped the google books ID anyhow.

But, instead of "journaltitle" and "langid" I'd like to have the traditional bibtex "journal" and "language" fields.

That's not traditional for biblatex though. Those are bibtex fields, for which biblatex provides fallback support. In any case:

if (Translator.BetterBibLaTeX) {
  if (this.has.journaltitle) this.has.journaltitle.name = 'journal'
  /* or less hacky:
  if (this.has.journaltitle) {
    this.add({ name: 'journal', value: this.has.journaltitle.value })
    this.remove('journaltitle')
  }
  */
  if (this.has.langid) this.has.langid.name = 'language'
}

Then, is there a way to replace typographic quotes without checking "export as plain-text latex commands"? I prefer unicode characters (of umlauts and accents etc.) to be preserved.

That's a reasonable request. Please open an issue on https://github.com/retorquere/zotero-better-bibtex/issues

And finally, I'm now getting capitalized strings enclosed in {{double braces}} – I prefer to protect strings manually, as websites are so inconsistent in how they display titles.

https://retorque.re/zotero-better-bibtex/faq/#bbt-is-changing-the-capitalization-of-my-titles--why has an explanation and a way to prevent case-meddling (which is a standard Zotero feature that BBT just happens to support).

Also, "booktitle" still doesn't get copied with the code you've provided above.

Sorry, that should have been

if (Translator.BetterBibLaTeX && item.itemType === 'book') this.add({ name: 'booktitle', value: item.title })