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.

src/i18n/locale.rs

@@ -0,0 +1,106 @@
+use gpui::Styled;
+use jiff::{Timestamp, tz::TimeZone};
+
+use crate::typography::format::{self, Separators};
+
+/// Which way a line of text runs.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum Direction {
+    Ltr,
+    Rtl,
+}
+
+impl Direction {
+    /// A flex row that starts at the reading side.
+    pub fn row<E: Styled>(self, element: E) -> E {
+        match self {
+            Direction::Ltr => element.flex().flex_row(),
+            Direction::Rtl => element.flex().flex_row_reverse(),
+        }
+    }
+
+    /// Text set against the reading side.
+    pub fn align<E: Styled>(self, element: E) -> E {
+        match self {
+            Direction::Ltr => element.text_left(),
+            Direction::Rtl => element.text_right(),
+        }
+    }
+}
+
+/// A language as it writes numbers, dates and lines.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct Locale {
+    pub tag: &'static str,
+    pub name: &'static str,
+    pub separators: Separators,
+    /// A numeric date, as `strftime` reads it.
+    pub date: &'static str,
+    pub direction: Direction,
+}
+
+const fn locale(
+    tag: &'static str,
+    name: &'static str,
+    (group, decimal): (char, char),
+    date: &'static str,
+    direction: Direction,
+) -> Locale {
+    Locale {
+        tag,
+        name,
+        separators: Separators { group, decimal },
+        date,
+        direction,
+    }
+}
+
+/// The locales Ely formats for, with CLDR's short numeric dates.
+pub const LOCALES: [Locale; 7] = [
+    locale(
+        "en-US",
+        "English (US)",
+        (',', '.'),
+        "%-m/%-d/%Y",
+        Direction::Ltr,
+    ),
+    locale(
+        "en-GB",
+        "English (UK)",
+        (',', '.'),
+        "%d/%m/%Y",
+        Direction::Ltr,
+    ),
+    locale("de-DE", "Deutsch", ('.', ','), "%d.%m.%Y", Direction::Ltr),
+    locale(
+        "fr-FR",
+        "Français",
+        ('\u{202f}', ','),
+        "%d/%m/%Y",
+        Direction::Ltr,
+    ),
+    locale("ja-JP", "日本語", (',', '.'), "%Y/%m/%d", Direction::Ltr),
+    locale("zh-CN", "中文", (',', '.'), "%Y/%-m/%-d", Direction::Ltr),
+    locale("he-IL", "עברית", (',', '.'), "%-d.%-m.%Y", Direction::Rtl),
+];
+
+impl Locale {
+    /// The locale with this tag; fails on one Ely does not list.
+    pub fn of(tag: &str) -> Locale {
+        *LOCALES
+            .iter()
+            .find(|locale| locale.tag == tag)
+            .unwrap_or_else(|| panic!("no locale {tag:?}"))
+    }
+
+    /// `value` with this locale's separators.
+    pub fn number(self, value: f64, decimals: usize) -> String {
+        format::number(value, decimals, self.separators)
+    }
+
+    /// The day `at` falls on in `zone`, written as this locale writes dates.
+    pub fn date(self, at: Timestamp, zone: &TimeZone) -> String {
+        format::datetime(at, zone, self.date)
+            .unwrap_or_else(|error| panic!("{}'s date pattern: {error}", self.tag))
+    }
+}