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.

examples/gallery/pages/mod.rs

@@ -18,6 +18,7 @@ mod finance;
 mod forms;
 mod generative;
 mod git;
+mod i18n;
 mod interaction;
 mod layout;
 mod lists;
@@ -92,8 +93,10 @@ pub const ALL: &[Page] = &[
     onboarding::PAGE,
     interaction::PAGE,
     theme::PAGE,
+    i18n::PAGE,
 ];
 
+pub use i18n::install as install_i18n;
 pub use interaction::bind_keys;
 pub use shell::{open_about, open_managed};