diff --git a/wp-admin/includes/class-language-pack-upgrader.php b/wp-admin/includes/class-language-pack-upgrader.php
index 855dbe642a..6f7cf742bd 100644
--- a/wp-admin/includes/class-language-pack-upgrader.php
+++ b/wp-admin/includes/class-language-pack-upgrader.php
@@ -332,26 +332,34 @@ class Language_Pack_Upgrader extends WP_Upgrader {
 		// Check that the folder contains a valid language.
 		$files = $wp_filesystem->dirlist( $remote_source );
 
-		// Check to see if a .po and .mo exist in the folder.
-		$po = false;
-		$mo = false;
+		// Check to see if the expected files exist in the folder.
+		$po  = false;
+		$mo  = false;
+		$php = false;
 		foreach ( (array) $files as $file => $filedata ) {
 			if ( str_ends_with( $file, '.po' ) ) {
 				$po = true;
 			} elseif ( str_ends_with( $file, '.mo' ) ) {
 				$mo = true;
+			} elseif ( str_ends_with( $file, '.l10n.php' ) ) {
+				$php = true;
 			}
 		}
 
+		if ( $php ) {
+			return $source;
+		}
+
 		if ( ! $mo || ! $po ) {
 			return new WP_Error(
 				'incompatible_archive_pomo',
 				$this->strings['incompatible_archive'],
 				sprintf(
-					/* translators: 1: .po, 2: .mo */
-					__( 'The language pack is missing either the %1$s or %2$s files.' ),
+					/* translators: 1: .po, 2: .mo, 3: .l10n.php */
+					__( 'The language pack is missing either the %1$s, %2$s, or %3$s files.' ),
 					'<code>.po</code>',
-					'<code>.mo</code>'
+					'<code>.mo</code>',
+					'<code>.l10n.php</code>'
 				)
 			);
 		}
diff --git a/wp-includes/l10n.php b/wp-includes/l10n.php
index 3b99ff848b..9e2a86a759 100644
--- a/wp-includes/l10n.php
+++ b/wp-includes/l10n.php
@@ -1505,25 +1505,25 @@ function get_available_languages( $dir = null ) {
  *
  * @since 3.7.0
  *
+ * @global WP_Textdomain_Registry $wp_textdomain_registry WordPress Textdomain Registry.
+ *
  * @param string $type What to search for. Accepts 'plugins', 'themes', 'core'.
  * @return array Array of language data.
  */
 function wp_get_installed_translations( $type ) {
+	global $wp_textdomain_registry;
+
 	if ( 'themes' !== $type && 'plugins' !== $type && 'core' !== $type ) {
 		return array();
 	}
 
-	$dir = 'core' === $type ? '' : "/$type";
+	$dir = 'core' === $type ? WP_LANG_DIR : WP_LANG_DIR . "/$type";
 
-	if ( ! is_dir( WP_LANG_DIR ) ) {
+	if ( ! is_dir( $dir ) ) {
 		return array();
 	}
 
-	if ( $dir && ! is_dir( WP_LANG_DIR . $dir ) ) {
-		return array();
-	}
-
-	$files = scandir( WP_LANG_DIR . $dir );
+	$files = $wp_textdomain_registry->get_language_files_from_path( $dir );
 	if ( ! $files ) {
 		return array();
 	}
@@ -1531,16 +1531,7 @@ function wp_get_installed_translations( $type ) {
 	$language_data = array();
 
 	foreach ( $files as $file ) {
-		if ( '.' === $file[0] || is_dir( WP_LANG_DIR . "$dir/$file" ) ) {
-			continue;
-		}
-		if ( ! str_ends_with( $file, '.po' ) ) {
-			continue;
-		}
-		if ( ! preg_match( '/(?:(.+)-)?([a-z]{2,3}(?:_[A-Z]{2})?(?:_[a-z0-9]+)?).po/', $file, $match ) ) {
-			continue;
-		}
-		if ( ! in_array( substr( $file, 0, -3 ) . '.mo', $files, true ) ) {
+		if ( ! preg_match( '/(?:(.+)-)?([a-z]{2,3}(?:_[A-Z]{2})?(?:_[a-z0-9]+)?)\.(?:mo|l10n\.php)/', basename( $file ), $match ) ) {
 			continue;
 		}
 
@@ -1548,7 +1539,25 @@ function wp_get_installed_translations( $type ) {
 		if ( '' === $textdomain ) {
 			$textdomain = 'default';
 		}
-		$language_data[ $textdomain ][ $language ] = wp_get_pomo_file_data( WP_LANG_DIR . "$dir/$file" );
+
+		if ( str_ends_with( $file, '.mo' ) ) {
+			$pofile = substr_replace( $file, '.po', - strlen( '.mo' ) );
+
+			if ( ! file_exists( $pofile ) ) {
+				continue;
+			}
+
+			$language_data[ $textdomain ][ $language ] = wp_get_pomo_file_data( $pofile );
+		} else {
+			$pofile = substr_replace( $file, '.po', - strlen( '.l10n.php' ) );
+
+			// If both a PO and a PHP file exist, prefer the PO file.
+			if ( file_exists( $pofile ) ) {
+				continue;
+			}
+
+			$language_data[ $textdomain ][ $language ] = wp_get_l10n_php_file_data( $file );
+		}
 	}
 	return $language_data;
 }
@@ -1578,6 +1587,41 @@ function wp_get_pomo_file_data( $po_file ) {
 	return $headers;
 }
 
+/**
+ * Extracts headers from a PHP translation file.
+ *
+ * @since 6.6.0
+ *
+ * @param string $php_file Path to a `.l10n.php` file.
+ * @return string[] Array of file header values keyed by header name.
+ */
+function wp_get_l10n_php_file_data( $php_file ) {
+	$data = (array) include $php_file;
+
+	unset( $data['messages'] );
+	$headers = array(
+		'POT-Creation-Date'  => 'pot-creation-date',
+		'PO-Revision-Date'   => 'po-revision-date',
+		'Project-Id-Version' => 'project-id-version',
+		'X-Generator'        => 'x-generator',
+	);
+
+	$result = array(
+		'POT-Creation-Date'  => '',
+		'PO-Revision-Date'   => '',
+		'Project-Id-Version' => '',
+		'X-Generator'        => '',
+	);
+
+	foreach ( $headers as $po_header => $php_header ) {
+		if ( isset( $data[ $php_header ] ) ) {
+			$result[ $po_header ] = $data[ $php_header ];
+		}
+	}
+
+	return $result;
+}
+
 /**
  * Displays or returns a Language selector.
  *
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 1dc961be77..09fb338a07 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.6-alpha-58061';
+$wp_version = '6.6-alpha-58062';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.