Looks alot like mozilla's project fluent, atleast in the basic use case.
I wonder why it hasn't been adopted more widely.
Here's a comparison between the two on Fluent's wiki: https://github.com/projectfluent/fluent/wiki/Fluent-and-ICU-...
It seems the last edit of the page was in 2019, so I'm not sure how up to date it is.
They seems to be a strong overlap of people behind both projects, so that likely explains the similarities.
I often wonder this myself, this really should be a standard by now.
Yes, Fluent informed much of the design of MessageFormat. See this FOSDEM talk: https://archive.fosdem.org/2023/schedule/event/mozilla_intme...