Parse Accept-Language header in wp_load_translations_early

src/wp-includes/l10n.php

@@ -2019,3 +2019,120 @@ function wp_get_word_count_type() {
 function has_translation( string $singular, string $textdomain = 'default', ?string $locale = null ): bool {
 	return WP_Translation_Controller::get_instance()->has_translation( $singular, $textdomain, $locale );
 }
+
+/**
+ * Parses the `Accept-Language` header to get a list of locales.
+ *
+ * The locales are returned in the format WordPress expects,
+ * so "fr-CH" becomes "fr_CH" and "fr" becomes "fr_FR".
+ *
+ * @link https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Accept-Language
+ *
+ * @since 6.7.0
+ * @access private
+ *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
+ * @return string[] Locales list.
+ */
+function get_locales_from_accept_language_header() {
+	global $wpdb;
+
+	if ( empty( $_SERVER['HTTP_ACCEPT_LANGUAGE'] ) || ! is_string( $_SERVER['HTTP_ACCEPT_LANGUAGE'] ) ) {
+		return array();
+	}
+
+	$locales = array();
+
+	$matches = array();
+	// Parses a header value such as "fr-CH, fr;q=0.9, en;q=0.8, de;q=0.7, *;q=0.5".
+	preg_match_all( '((?P<code>[a-z-_A-Z]+|\*)([;q=]+?(?P<prio>1|0\.\d))?)', $_SERVER['HTTP_ACCEPT_LANGUAGE'], $matches );
+
+	if ( empty( $matches['code'] ) ) {
+		return $locales;
+	}
+
+	$codes = $matches['code'];
+
+	// An empty priority defaults to 1.
+	$prios = array_map(
+		static function ( $value ) {
+			if ( '' === $value ) {
+				return 1.0;
+			}
+
+			return (float) $value;
+		},
+		$matches['prio']
+	);
+
+	// Sort codes by priority.
+	usort(
+		$codes,
+		static function ( $a, $b ) use ( $codes, $prios ) {
+			$index_a = array_search( $a, $codes, true );
+			$index_b = array_search( $b, $codes, true );
+
+			return $prios[ $index_b ] <=> $prios[ $index_a ];
+		}
+	);
+
+	$translations = array();
+
+	/*
+	 * Get list of available translations without potentially deleting an expired transient and causing an HTTP request.
+	 * Only works if either the object cache or the database are already available.
+	 */
+	if ( wp_using_ext_object_cache() ) {
+		wp_start_object_cache();
+		$translations = wp_cache_get( 'available_translations', 'site-transient' );
+	} elseif ( isset( $wpdb ) ) {
+		$translations = get_site_option( '_site_transient_available_translations' );
+	}
+
+	$has_available_translations = is_array( $translations ) && ! empty( $translations );
+
+	foreach ( $codes as $code ) {
+		if ( '*' === $code ) {
+			// Ignore anything after the wildcard, as we can then just default to en_US.
+			break;
+		}
+
+		$locale = sanitize_locale_name( str_replace( '-', '_', $code ) );
+
+		if ( '' === $locale ) {
+			continue;
+		}
+
+		// If English is accepted, then there is no point in adding any other locales after it.
+		if ( 'en' === $locale ) {
+			break;
+		}
+
+		if ( $has_available_translations ) {
+			$found = array_keys(
+				array_filter(
+					$translations,
+					static function ( $translation ) use ( $locale, $code ) {
+						return $locale === $translation['language'] || in_array( $code, $translation['iso'], true );
+					}
+				)
+			);
+			sort( $found );
+
+			if ( ! empty( $found ) ) {
+				array_push( $locales, ...$found );
+			}
+		} else {
+
+			$locales[] = $locale;
+
+			// Fallback approximation, supporting cases like "el", but also "fr" -> "fr_FR",
+			if ( 2 === strlen( $locale ) ) {
+				$locales[] = $locale . '_' . strtoupper( $locale );
+			}
+		}
+	}
+
+	return $locales;
+}

src/wp-includes/load.php

@@ -1550,6 +1550,9 @@ function wp_load_translations_early() {
 	require_once ABSPATH . WPINC . '/class-wp-locale.php';
 	require_once ABSPATH . WPINC . '/class-wp-locale-switcher.php';
 
+	// For sanitize_locale_name().
+	require_once ABSPATH . WPINC . '/formatting.php';
+
 	// General libraries.
 	require_once ABSPATH . WPINC . '/plugin.php';
 
@@ -1572,6 +1575,9 @@ function wp_load_translations_early() {
 			$locales[] = $wp_local_package;
 		}
 
+		// Try the browser's locale
+		$locales = array_merge( $locales, get_locales_from_accept_language_header() );
+
 		if ( ! $locales ) {
 			break;
 		}

tests/e2e/specs/maintenance-mode.test.js

@@ -24,10 +24,23 @@ test.describe( 'Maintenance mode', () => {
 		unlinkSync( maintenanceLockFile );
 	} );
 
-	test( 'should display maintenance mode page', async ( { page } ) => {
-		await page.goto( '/' );
-		await expect(
-			page.getByText( /Briefly unavailable for scheduled maintenance\. Check back in a minute\./ )
-		).toBeVisible();
+	test.describe( 'Default (en_US)', () => {
+		test( 'should display maintenance mode page', async ( { page } ) => {
+			await page.goto( '/' );
+			await expect(
+				page.getByText( /Briefly unavailable for scheduled maintenance\. Check back in a minute\./ )
+			).toBeVisible();
+		} );
+	} );
+
+	test.describe( 'Localized (de_DE)', () => {
+		test.use( { locale: 'de-DE' } ); // Sets the Accept-Language header.
+
+		test( 'should display maintenance mode page', async ( { page } ) => {
+			await page.goto( '/' );
+			await expect(
+				page.getByText( /Wegen Wartungsarbeiten ist diese Website kurzzeitig nicht verfügbar/ )
+			).toBeVisible();
+		} );
 	} );
 } );

tests/phpunit/tests/l10n.php

@@ -630,4 +630,163 @@ public function test_length_of_comment_excerpt_should_be_counted_by_chars_in_Jap
 
 		$this->assertSame( $expect, $comment_excerpt );
 	}
+
+	/**
+	 * @ticket 30049
+	 *
+	 * @covers ::get_locales_from_accept_language_header
+	 *
+	 * @dataProvider data_get_locales_from_accept_language_header
+	 */
+	public function test_get_locales_from_accept_language_header( $input, $expected, $has_transient = false ) {
+		if ( 'missing' === $input ) {
+			unset( $_SERVER['HTTP_ACCEPT_LANGUAGE'] );
+		} else {
+			$_SERVER['HTTP_ACCEPT_LANGUAGE'] = $input;
+		}
+
+		if ( $has_transient ) {
+			// Fetches available translations and stores them in a transient.
+			require_once ABSPATH . 'wp-admin/includes/translation-install.php';
+
+			wp_get_available_translations();
+		} else {
+			delete_site_transient( 'available_translations' );
+			wp_cache_delete( 'available_translations', 'site-transient' );
+		}
+
+		$actual = get_locales_from_accept_language_header();
+
+		unset( $_SERVER['HTTP_ACCEPT_LANGUAGE'] );
+
+		$this->assertSame( $expected, $actual, 'Parsed locales list does not expect actual list' );
+	}
+
+	public static function data_get_locales_from_accept_language_header() {
+		return array(
+			'Missing header'                          => array(
+				'missing', // Will be handled specially in the test.
+				array(),
+			),
+			'Null header'                             => array(
+				null,
+				array(),
+			),
+			'Empty header'                            => array(
+				false,
+				array(),
+			),
+			'Invalid type'                            => array(
+				array(),
+				array(),
+			),
+			'Wildcard'                                => array(
+				'*',
+				array(),
+			),
+			'Two-letter locales'                      => array(
+				'de, fr, el, bn, ky',
+				array(
+					'de',
+					'de_DE',
+					'fr',
+					'fr_FR',
+					'el',
+					'el_EL',
+					'bn',
+					'bn_BN',
+					'ky',
+					'ky_KY',
+				),
+			),
+			'Two-letter locales, with transient'      => array(
+				'de, fr, el, bn, ky',
+				array(
+					'de_AT',
+					'de_CH',
+					'de_CH_informal',
+					'de_DE',
+					'de_DE_formal',
+					'fr_BE',
+					'fr_CA',
+					'fr_FR',
+					'el',
+					'bn_BD',
+					'kir',
+				),
+				true,
+			),
+			// en-GB-oxendict is supported by Chrome, ca-valencia by Firefox.
+			'Longer variants'                         => array(
+				'en-GB-oxendict, de-DE, ca-valencia, ca',
+				array(
+					'en_GB_oxendict',
+					'de_DE',
+					'ca_valencia',
+					'ca',
+					'ca_CA',
+				),
+			),
+			'Longer variants, with transient'         => array(
+				'en-GB-oxendict, de-DE, ca-valencia, ca',
+				array(
+					'de_DE',
+					// Absent because there is no language pack for ca_valencia yet, see https://make.wordpress.org/polyglots/teams/.
+					//'ca_valencia',
+					'ca',
+				),
+				true,
+			),
+			'Multiple types, weighed'                 => array(
+				'fr-BE, fr;q=0.7, en;q=0.6, de;q=0.8, es-ES;q=0.5',
+				array(
+					'fr_BE',
+					'de',
+					'de_DE',
+					'fr',
+					'fr_FR',
+				),
+			),
+			'Multiple types, weighed, with transient' => array(
+				'fr-BE, fr;q=0.7, en;q=0.6, de;q=0.8, es-ES;q=0.5',
+				array(
+					'fr_BE',
+					'de_AT',
+					'de_CH',
+					'de_CH_informal',
+					'de_DE',
+					'de_DE_formal',
+					'fr_BE',
+					'fr_CA',
+					'fr_FR',
+				),
+				true,
+			),
+			'Multiple types, weighed, with wildcard'  => array(
+				'fr-BE, fr;q=0.7, *;q=0.6, de;q=0.8, es-ES;q=0.5',
+				array(
+					'fr_BE',
+					'de',
+					'de_DE',
+					'fr',
+					'fr_FR',
+				),
+			),
+			'Multiple types, weighed, with wildcard, with transient' => array(
+				'fr-BE, fr;q=0.7, *;q=0.6, de;q=0.8, es-ES;q=0.5',
+				array(
+					'fr_BE',
+					'de_AT',
+					'de_CH',
+					'de_CH_informal',
+					'de_DE',
+					'de_DE_formal',
+					'fr_BE',
+					'fr_CA',
+					'fr_FR',
+				),
+				true,
+			),
+		);
+	}
 }