From fd49a10f1c02d4e44647c74d4799f191f8601693 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ky=E2=84=93e=20Hensel?= <k-yle@users.noreply.github.com>
Date: Tue, 25 Mar 2025 00:27:18 +1100
Subject: [PATCH] consider language scripts when determining the locale to use
 (#10910)

---
 CHANGELOG.md                |  2 ++
 modules/core/localizer.js   | 30 ++++++++++++++++++++----------
 test/spec/core/localizer.js | 25 +++++++++++++++++++++++++
 3 files changed, 47 insertions(+), 10 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 98a23acad..410b0db2b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -57,6 +57,7 @@ _Breaking developer changes, which may affect downstream projects or sites that
 * Make features clickable when "Full Fill" rendering style is selected
 * Fix calculation of access field placeholders for multi selections ([#9333])
 #### :earth_asia: Localization
+* Consider language scripts when determining the locale to use ([#10910], thanks [@k-yle])
 #### :hourglass: Performance
 #### :mortar_board: Walkthrough / Help
 #### :rocket: Presets
@@ -74,6 +75,7 @@ _Breaking developer changes, which may affect downstream projects or sites that
 [#10843]: https://github.com/openstreetmap/iD/pull/10843
 [#10852]: https://github.com/openstreetmap/iD/issues/10852
 [#10885]: https://github.com/openstreetmap/iD/issues/10885
+[#10910]: https://github.com/openstreetmap/iD/pull/10910
 [id-tagging-schema#609]: https://github.com/openstreetmap/id-tagging-schema/issues/609
 [@0xatulpatil]: https://github.com/0xatulpatil
 [@MohamedAli00949]: https://github.com/MohamedAli00949
diff --git a/modules/core/localizer.js b/modules/core/localizer.js
index 1637c5d86..75616032e 100644
--- a/modules/core/localizer.js
+++ b/modules/core/localizer.js
@@ -108,11 +108,8 @@ export function coreLocalizer() {
                 _dataLocales = results[1];
 
                 let indexes = results.slice(2);
-                let requestedLocales = (_preferredLocaleCodes || [])
-                    .concat(utilDetect().browserLocales)   // List of locales preferred by the browser in priority order.
-                    .concat(['en']);   // fallback to English since it's the only guaranteed complete language
 
-                _localeCodes = localesToUseFrom(requestedLocales);
+                _localeCodes = localizer.localesToUseFrom(_dataLocales);
                 _localeCode = _localeCodes[0];   // Run iD in the highest-priority locale; the rest are fallbacks
 
                 let loadStringsPromises = [];
@@ -139,23 +136,36 @@ export function coreLocalizer() {
     };
 
     // Returns the locales from `requestedLocales` supported by iD that we should use
-    function localesToUseFrom(requestedLocales) {
-        let supportedLocales = _dataLocales;
+    /** @param {{ [locale: string]: unknown }} supportedLocales */
+    localizer.localesToUseFrom = (supportedLocales) => {
+        const requestedLocales = [
+          ...(_preferredLocaleCodes || []),
+          ...utilDetect().browserLocales,  // List of locales preferred by the browser in priority order.
+          'en',  // fallback to English since it's the only guaranteed complete language
+        ];
 
+        /** @type {string[]} */
         let toUse = [];
-        for (let i in requestedLocales) {
-            let locale = requestedLocales[i];
+        for (const locale of requestedLocales) {
             if (supportedLocales[locale]) toUse.push(locale);
 
-            if (locale.includes('-')) {
+            if ('Intl' in window && 'Locale' in window.Intl) {
                 // Full locale ('es-ES'), add fallback to the base ('es')
+                const localeObj = new Intl.Locale(locale);
+                const withoutScript = `${localeObj.language}-${localeObj.region}`;
+                const base = localeObj.language;
+
+                if (supportedLocales[withoutScript]) toUse.push(withoutScript);
+                if (supportedLocales[base]) toUse.push(base);
+            } else if (locale.includes('-')) {
+                // legacy logic: if Intl.Locale is not available
                 let langPart = locale.split('-')[0];
                 if (supportedLocales[langPart]) toUse.push(langPart);
             }
         }
         // remove duplicates
         return utilArrayUniq(toUse);
-    }
+    };
 
     function updateForCurrentLocale() {
         if (!_localeCode) return;
diff --git a/test/spec/core/localizer.js b/test/spec/core/localizer.js
index de157710a..690c69444 100644
--- a/test/spec/core/localizer.js
+++ b/test/spec/core/localizer.js
@@ -90,4 +90,29 @@ describe('iD.coreLocalizer', function() {
             expect(countDecimalPlaces('10')).to.eql(0);
         });
     });
+
+    describe('localesToUseFrom', () => {
+        const SUPPORTED_LANGS = {
+            en: true,
+            'en-AU': true,
+            fr: true,
+            zh: true,
+            'zh-CN': true,
+        };
+        it.each([
+            /* [requested, matching] */
+            [[], ['en']],
+            [['en'], ['en']],
+            [['en-AU'], ['en-AU', 'en']],
+            [['zh'], ['zh', 'en']],
+            [['zh-CN'], ['zh-CN', 'zh', 'en']],
+            [['zh-Hans-CN'], ['zh-CN', 'zh', 'en']],
+            [['zh-Hans'], ['zh', 'en']],
+            [['fr-Latn'], ['fr', 'en']],
+        ])('resolves %s to %s', (requested, matching) => {
+            const localiser = iD.coreLocalizer();
+            localiser.preferredLocaleCodes(requested);
+            expect(localiser.localesToUseFrom(SUPPORTED_LANGS)).toStrictEqual(matching);
+        });
+    });
 });