Fix invalid locale issue (#2548)

* fix: fallback to default locale in project without a root locale for pages not matching the default locale slug

* Update changeset

* refactor: remove duplicated `slugToLocale()`

---------

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

6 files changed, 49 insertions, 25 deletions

diff --git a/.changeset/great-toys-train.md b/.changeset/great-toys-train.md
new file mode 100644
index 00000000..0bef54b1
--- /dev/null
+++ b/.changeset/great-toys-train.md
@@ -0,0 +1,5 @@
+---
+'@astrojs/starlight': patch
+---
+
+Fixes a URL localization edge case. In projects without a root locale configured, slugs without a locale prefix did not fall back to the default locale as expected.
diff --git a/packages/starlight/__tests__/i18n-non-root-single-locale/routing.test.ts b/packages/starlight/__tests__/i18n-non-root-single-locale/routing.test.ts
index 5eae14d8..fae14a98 100644
--- a/packages/starlight/__tests__/i18n-non-root-single-locale/routing.test.ts
+++ b/packages/starlight/__tests__/i18n-non-root-single-locale/routing.test.ts
@@ -7,6 +7,7 @@ vi.mock('astro:content', async () =>
 			['fr/index.mdx', { title: 'Accueil' }],
 			// @ts-expect-error — Using a slug not present in Starlight docs site
 			['en/index.mdx', { title: 'Home page' }],
+			['404.md', { title: '404' }],
 		],
 	})
 );
@@ -17,16 +18,16 @@ test('route slugs are normalized', () => {
 });
 
 test('routes for the configured locale have locale data added', () => {
-	for (const route of routes) {
-		if (route.id.startsWith('fr')) {
-			expect(route.lang).toBe('fr-CA');
-			expect(route.dir).toBe('ltr');
-			expect(route.locale).toBe('fr');
-		} else {
-			expect(route.lang).toBe('fr-CA');
-			expect(route.dir).toBe('ltr');
-			expect(route.locale).toBeUndefined();
-		}
+	expect(routes[0]?.lang).toBe('fr-CA');
+	expect(routes[0]?.dir).toBe('ltr');
+	expect(routes[0]?.locale).toBe('fr');
+});
+
+test('routes not matching the configured locale fall back to the default locale', () => {
+	for (const route of routes.slice(1)) {
+		expect(route.lang).toBe('fr-CA');
+		expect(route.dir).toBe('ltr');
+		expect(route.locale).toBe('fr');
 	}
 });
 
diff --git a/packages/starlight/__tests__/i18n/routing.test.ts b/packages/starlight/__tests__/i18n/routing.test.ts
index b854794b..8c025e2c 100644
--- a/packages/starlight/__tests__/i18n/routing.test.ts
+++ b/packages/starlight/__tests__/i18n/routing.test.ts
@@ -14,6 +14,8 @@ vi.mock('astro:content', async () =>
 			['en/guides/authoring-content.md', { title: 'Création de contenu en Markdown' }],
 			// @ts-expect-error — Using a slug not present in Starlight docs site
 			['en/404.md', { title: 'Page introuvable' }],
+			['it/index.mdx', { title: 'Pagina iniziale' }],
+			['404.md', { title: '404' }],
 		],
 	})
 );
@@ -42,6 +44,14 @@ test('routes have locale data added', () => {
 			expect(lang).toBe('fr');
 			expect(dir).toBe('ltr');
 			expect(locale).toBe('fr');
+		} else if (id.startsWith('pt-br')) {
+			expect(lang).toBe('pt-BR');
+			expect(dir).toBe('ltr');
+			expect(locale).toBe('pt-br');
+		} else {
+			expect(lang).toBe('en-US');
+			expect(dir).toBe('ltr');
+			expect(locale).toBe('en');
 		}
 	}
 });
diff --git a/packages/starlight/integrations/shared/pathToLocale.ts b/packages/starlight/integrations/shared/pathToLocale.ts
index 5c229a47..2ecb997c 100644
--- a/packages/starlight/integrations/shared/pathToLocale.ts
+++ b/packages/starlight/integrations/shared/pathToLocale.ts
@@ -1,14 +1,6 @@
 import type { AstroConfig } from 'astro';
 import type { StarlightConfig } from '../../types';
-
-function slugToLocale(
-	slug: string | undefined,
-	localesConfig: StarlightConfig['locales']
-): string | undefined {
-	const locales = Object.keys(localesConfig || {});
-	const baseSegment = slug?.split('/')[0];
-	return baseSegment && locales.includes(baseSegment) ? baseSegment : undefined;
-}
+import { slugToLocale } from './slugToLocale';
 
 /** Get current locale from the full file path. */
 export function pathToLocale(
@@ -17,7 +9,7 @@ export function pathToLocale(
 		starlightConfig,
 		astroConfig,
 	}: {
-		starlightConfig: { locales: StarlightConfig['locales'] };
+		starlightConfig: Pick<StarlightConfig, 'defaultLocale' | 'locales'>;
 		astroConfig: { root: AstroConfig['root']; srcDir: AstroConfig['srcDir'] };
 	}
 ): string | undefined {
@@ -31,5 +23,5 @@ export function pathToLocale(
 	// Strip docs path leaving only content collection file ID.
 	// Example: /Users/houston/repo/src/content/docs/en/guide.md => en/guide.md
 	const slug = path?.replace(docsDir.pathname, '');
-	return slugToLocale(slug, starlightConfig.locales);
+	return slugToLocale(slug, starlightConfig);
 }
diff --git a/packages/starlight/integrations/shared/slugToLocale.ts b/packages/starlight/integrations/shared/slugToLocale.ts
new file mode 100644
index 00000000..69e1f3bc
--- /dev/null
+++ b/packages/starlight/integrations/shared/slugToLocale.ts
@@ -0,0 +1,18 @@
+import type { StarlightConfig } from '../../types';
+
+/**
+ * Get the “locale” of a slug. This is the base path at which a language is served.
+ * For example, if French docs are in `src/content/docs/french/`, the locale is `french`.
+ * Root locale slugs will return `undefined`.
+ * @param slug A collection entry slug
+ */
+export function slugToLocale(
+	slug: string | undefined,
+	config: Pick<StarlightConfig, 'defaultLocale' | 'locales'>
+): string | undefined {
+	const localesConfig = config.locales ?? {};
+	const baseSegment = slug?.split('/')[0];
+	if (baseSegment && localesConfig[baseSegment]) return baseSegment;
+	if (!localesConfig.root) return config.defaultLocale.locale;
+	return undefined;
+}
diff --git a/packages/starlight/utils/slugs.ts b/packages/starlight/utils/slugs.ts
index afe971ba..e74d6841 100644
--- a/packages/starlight/utils/slugs.ts
+++ b/packages/starlight/utils/slugs.ts
@@ -1,6 +1,7 @@
 import config from 'virtual:starlight/user-config';
 import { BuiltInDefaultLocale } from './i18n';
 import { stripTrailingSlash } from './path';
+import { slugToLocale as getLocaleFromSlug } from '../integrations/shared/slugToLocale';
 
 export interface LocaleData {
 	/** Writing direction. */
@@ -18,10 +19,7 @@ export interface LocaleData {
  * @param slug A collection entry slug
  */
 function slugToLocale(slug: string): string | undefined {
-	const locales = Object.keys(config.locales || {});
-	const baseSegment = slug.split('/')[0];
-	if (baseSegment && locales.includes(baseSegment)) return baseSegment;
-	return undefined;
+	return getLocaleFromSlug(slug, config);
 }
 
 /** Get locale information for a given slug. */