CSL currently has no fields for transliteration, so you should just include the transliteration in the regular fields. Having said that, there is a CSL variant called CSL-M (https://citeproc-js.readthedocs.io/en/latest/csl-m/) that, among other things, offers special support for multilingual demands. Currently, CSL-M is only supported by citeproc-js, so you can use those features with the Zotero fork Juris-M, and also with upstream Zotero (Zotero will complain about styles using these features, but they will work nevertheless). Pandoc-citeproc does not understand CSL-M, but citeproc-rs, a new citeproc under development (https://github.com/cormacrelf/citeproc-rs), might bring those features also to pandoc.