I think this is just the format and specification itself, language selection and file storage and the like will depend on an implementing library. The i18next version for example (bizarrely) puts the whole string in a JSON key, but to be honest I think this is a bad example: https://github.com/i18next/i18next-icu?tab=readme-ov-file#mo...

Here is a proposal for a message resource format on top of MF2.0 - https://github.com/eemeli/message-resource-wg