Question: Are there 2 letter ISO codes for the pinyin or hepburn transliterations? If not, are there non-ISO abbreviations in common use? Thanks.

Answer: 
ISO has codes for languages ([ISO 639](https://en.wikipedia.org/wiki/ISO_639-2)), and for scripts ([ISO 15924](https://en.wikipedia.org/wiki/ISO_15924)); but it has no codes for transliterations, as you can see by perusing ISO's standards on [Writing and Transliteration](https://www.iso.org/ics/01.140.10/x/). ISO adopts and standardises transliterations; but unlike languages and scripts, it has not catalogued them.

Using existing ISO codes, *zh-Latn* means "Chinese in Latin Script", and *zh-CN-Latn* means "Chinese in Latin Script, localised to China"; but that just implies "Pinyin" (and what country would we attribute Hepburn to?) It is not a solution.

I can't find evidence that there are standard codes for transliterations anywhere else either. A 2011 [IETF RFC draft for transliteration codes](https://datatracker.ietf.org/doc/html/draft-falk-transliteration-tags-01) went nowhere.
ISO has some standard romanization systems listed at <https://en.wikipedia.org/wiki/List_of_ISO_romanizations>. Pinyin is **ISO 7098**, but unfortunately no other systems of Chinese romanizations have been given ISO codes so you may not find this useful.