The server reads messages/<locale>.json from disk on first use. A missing
or corrupt file threw, and since the matchers iterate every locale, one bad
non-English catalog turned every request that touched them into a 500.
That locale is now served the English catalog, and the failure is logged
once as i18n.catalog.fallback with the locale and the reason. English is
the fallback itself: a process whose messages/en.json is missing or does
not parse refuses to start, with the file and the reason in the message.
The image build checks the catalogs twice with
scripts/verify-message-catalogs.mjs: in the builder against the locale
list in the source, and in the runner stage against the copies the image
carries.
