i18n: a locale per app, catalogs of messages, mirrored stacks

i18n::I18n is a gpui global holding the shown locale and one catalog of
messages per locale. text fills each {name} from its arguments; a
missing key, argument or catalog fails, and every catalog holds the
first one's keys. Locale writes numbers with its separators and dates
in CLDR's short numeric form; Hebrew replaces Arabic, whose Egyptian
digits Ely does not write. Direction::row mirrors a stack and
Direction::align sets text against the reading side. The gallery's
chapter 40 shows the messages in seven locales, every locale's number
and date, and a file list that mirrors under Hebrew.

tasks/ch31-43.md

@@ -273,10 +273,10 @@
 
 ## T40a i18n — `src/i18n`
 
-- [ ] I18nProvider
-- [ ] Trans / LocalizedText
-- [ ] LocaleNumber / LocaleDate
-- [ ] RTL layout — prove: mirrored stacks
+- [x] I18nProvider → `i18n::I18n`, a gpui global: the shown locale and one catalog per locale
+- [x] Trans / LocalizedText → `I18n::text`, a message with its `{name}` arguments filled
+- [x] LocaleNumber / LocaleDate → `Locale::number` and `Locale::date`, CLDR's separators and short dates
+- [x] RTL layout — proven: `Direction::row` mirrors a stack and `Direction::align` sets its text's side; Ely's other components stay left to right
 
 ## T40b a11y — `src/i18n`