Isn't this literally what locales are for? Instead of "English" you have "En-US", "En-UK", "En-Ind" etc.

Locales can help distinguish country-specific variants (e.g. US vs. UK spelling), but pronunciation varies a lot at the sub-national level. If you want to support all possible accents, you'll need a more fine-grained encoding.

I think locales are for dialects, where you can have different terms used for the same concept. Here you can have someone speak the en-NZ dialect, but with a French accent.

Also, we would need en-FR, en-ES, en-IT, etc. All languages as spoken from all other native languages. And obviously the strength of the accent varies.

Yeah, also a Moravian (ancient nation united with the Czechs more than 1000 years ago) person will speak English differently than a Czech (as in nation, not state) person even though we all speak the Czech language.

A solution could be to tag both the dialect and the accent with language codes. Native speakers of Moravian Czech will probably have similar accents when they speak New Zealand English. Using Glottolog IDs as tags for example, this might be represented as { dialect:"newz1240", accent:"czec1259" }. If the program can already recognise the New Zealand English dialect and the Moravian Czech accent, it might then leverage both of those to recognise the speech of a Moravian person speaking New Zealand English.