feat(i18n): an application uses Core's translator and words Core's messages its own way

The ./i18n entry exports t, tn, msg, translateKnown and the number and date
formatters. setMessageOverrides({ en: {...}, de: {...} }) shows the
application's text for any catalog key: in the active locale, else its
English, else Core's translation.

changelog.d/1741-message-overrides.md

@@ -0,0 +1,6 @@
+- Let an application that composes Core use its translator and word Core's
+  messages its own way. The `./i18n` entry exports `t`, `tn`, `msg`,
+  `translateKnown` and the number and date formatters, and
+  `setMessageOverrides({ en: {…}, de: {…} })` shows the application's text for
+  any catalog key, in the active locale or else in English (samehkhamis,
+  #1741).