Fix translation issue for languages with a region subtag (#2757)

Co-authored-by: Chris Swithinbank <swithinbank@gmail.com>

7 files changed, 47 insertions, 23 deletions

diff --git a/.changeset/brave-kings-vanish.md b/.changeset/brave-kings-vanish.md
new file mode 100644
index 00000000..22a63b76
--- /dev/null
+++ b/.changeset/brave-kings-vanish.md
@@ -0,0 +1,5 @@
+---
+'@astrojs/starlight': patch
+---
+
+Fixes a UI string translation issue for languages with a region subtag.
diff --git a/packages/starlight/__tests__/test-utils.ts b/packages/starlight/__tests__/test-utils.ts
index d6e3a711..12ebc804 100644
--- a/packages/starlight/__tests__/test-utils.ts
+++ b/packages/starlight/__tests__/test-utils.ts
@@ -52,8 +52,9 @@ function mockDoc(
 
 function mockDict(id: string, data: z.input<ReturnType<typeof i18nSchema>>) {
 	return {
-		id,
+		id: project.legacyCollections ? id : id.toLocaleLowerCase(),
 		data: i18nSchema().parse(data),
+		filePath: project.legacyCollections ? undefined : `src/content/i18n/${id}.yml`,
 	};
 }
 
diff --git a/packages/starlight/utils/navigation.ts b/packages/starlight/utils/navigation.ts
index 0944eb5d..37184627 100644
--- a/packages/starlight/utils/navigation.ts
+++ b/packages/starlight/utils/navigation.ts
@@ -13,7 +13,12 @@ import type {
 import { createPathFormatter } from './createPathFormatter';
 import { formatPath } from './format-path';
 import { BuiltInDefaultLocale, pickLang } from './i18n';
-import { ensureLeadingSlash, ensureTrailingSlash, stripLeadingAndTrailingSlashes } from './path';
+import {
+	ensureLeadingSlash,
+	ensureTrailingSlash,
+	stripExtension,
+	stripLeadingAndTrailingSlashes,
+} from './path';
 import { getLocaleRoutes, routes, type Route } from './routing';
 import { localeToLang, localizedId, slugToPathname } from './slugs';
 import type { StarlightConfig } from './user-config';
@@ -510,12 +515,6 @@ function applyPrevNextLinkConfig(
 	return paginationEnabled ? link : undefined;
 }
 
-/** Remove the extension from a path. */
-function stripExtension(path: string) {
-	const periodIndex = path.lastIndexOf('.');
-	return path.slice(0, periodIndex > -1 ? periodIndex : undefined);
-}
-
 /** Get a sidebar badge for a given item. */
 function getSidebarBadge(
 	config: I18nBadgeConfig,
diff --git a/packages/starlight/utils/path.ts b/packages/starlight/utils/path.ts
index 68b1f5c2..cf30d038 100644
--- a/packages/starlight/utils/path.ts
+++ b/packages/starlight/utils/path.ts
@@ -50,3 +50,9 @@ export function ensureHtmlExtension(path: string) {
 	}
 	return ensureLeadingSlash(path);
 }
+
+/** Remove the extension from a path. */
+export function stripExtension(path: string) {
+	const periodIndex = path.lastIndexOf('.');
+	return path.slice(0, periodIndex > -1 ? periodIndex : undefined);
+}
diff --git a/packages/starlight/utils/translations.ts b/packages/starlight/utils/translations.ts
index 499ded29..71ce6e72 100644
--- a/packages/starlight/utils/translations.ts
+++ b/packages/starlight/utils/translations.ts
@@ -1,15 +1,20 @@
 import { getCollection, type CollectionEntry, type DataCollectionKey } from 'astro:content';
 import config from 'virtual:starlight/user-config';
+import project from 'virtual:starlight/project-context';
 import pluginTranslations from 'virtual:starlight/plugin-translations';
 import type { i18nSchemaOutput } from '../schemas/i18n';
 import { createTranslationSystem } from './createTranslationSystem';
 import type { RemoveIndexSignature } from './types';
+import { getCollectionPathFromRoot } from './collection';
+import { stripExtension, stripLeadingSlash } from './path';
 
 // @ts-ignore - This may be a type error in projects without an i18n collection and running
 // `tsc --noEmit` in their project. Note that it is not possible to inline this type in
 // `UserI18nSchema` because this would break types for users having multiple data collections.
 type i18nCollection = CollectionEntry<'i18n'>;
 
+const i18nCollectionPathFromRoot = getCollectionPathFromRoot('i18n', project);
+
 export type UserI18nSchema = 'i18n' extends DataCollectionKey
 	? i18nCollection extends { data: infer T }
 		? i18nSchemaOutput & T
@@ -17,14 +22,20 @@ export type UserI18nSchema = 'i18n' extends DataCollectionKey
 	: i18nSchemaOutput;
 export type UserI18nKeys = keyof RemoveIndexSignature<UserI18nSchema>;
 
-/** Get all translation data from the i18n collection, keyed by `id`, which matches locale. */
+/** Get all translation data from the i18n collection, keyed by `lang`, which are BCP-47 language tags. */
 async function loadTranslations() {
 	// Briefly override `console.warn()` to silence logging when a project has no i18n collection.
 	const warn = console.warn;
 	console.warn = () => {};
 	const userTranslations: Record<string, UserI18nSchema> = Object.fromEntries(
 		// @ts-ignore — may be a type error in projects without an i18n collection
-		(await getCollection('i18n')).map(({ id, data }) => [id, data] as const)
+		(await getCollection('i18n')).map(({ id, data, filePath }) => {
+			const lang =
+				project.legacyCollections || !filePath
+					? id
+					: stripExtension(stripLeadingSlash(filePath.replace(i18nCollectionPathFromRoot, '')));
+			return [lang, data] as const;
+		})
 	);
 	// Restore the original warn implementation.
 	console.warn = warn;
diff --git a/packages/starlight/virtual-internal.d.ts b/packages/starlight/virtual-internal.d.ts
index 3c4d5ef9..10292513 100644
--- a/packages/starlight/virtual-internal.d.ts
+++ b/packages/starlight/virtual-internal.d.ts
@@ -1,16 +1,3 @@
-declare module 'virtual:starlight/project-context' {
-	const ProjectContext: {
-		root: string;
-		srcDir: string;
-		trailingSlash: import('astro').AstroConfig['trailingSlash'];
-		build: {
-			format: import('astro').AstroConfig['build']['format'];
-		};
-		legacyCollections: boolean;
-	};
-	export default ProjectContext;
-}
-
 declare module 'virtual:starlight/git-info' {
 	export function getNewestCommitDate(file: string): Date;
 }
diff --git a/packages/starlight/virtual.d.ts b/packages/starlight/virtual.d.ts
index 17ed33d5..5ff0c53c 100644
--- a/packages/starlight/virtual.d.ts
+++ b/packages/starlight/virtual.d.ts
@@ -7,3 +7,18 @@ declare module 'virtual:starlight/plugin-translations' {
 	const PluginTranslations: import('./utils/plugins').PluginTranslations;
 	export default PluginTranslations;
 }
+
+// TODO: Move back to `virtual-internal.d.ts` when possible. For example, when dropping support for
+// legacy collections, `utils/translations.ts` would no longer need to import project context.
+declare module 'virtual:starlight/project-context' {
+	const ProjectContext: {
+		root: string;
+		srcDir: string;
+		trailingSlash: import('astro').AstroConfig['trailingSlash'];
+		build: {
+			format: import('astro').AstroConfig['build']['format'];
+		};
+		legacyCollections: boolean;
+	};
+	export default ProjectContext;
+}