We recently had a contribution to enable messages in theme resources [1].
Messages from theme resources are loaded before any messages from themes
and are available to all themes.
This would make it easy to create a built-in theme resource with shared
messages between all themes.
Even better we can extract shared messages from the English message bundle.
Then other translations can be done later by the community as wanted.
Anyone interested in picking this work up?
[1]
https://issues.jboss.org/browse/KEYCLOAK-8818