diff --git a/src/wp-includes/ms-site.php b/src/wp-includes/ms-site.php
index ddc71380b3..b7a14c47f8 100644
--- a/src/wp-includes/ms-site.php
+++ b/src/wp-includes/ms-site.php
@@ -52,18 +52,12 @@ function wp_insert_site( array $data ) {
 		'lang_id'      => 0,
 	);
 
-	// Extract the passed arguments that may be relevant for site initialization.
-	$args = array_diff_key( $data, $defaults );
-	if ( isset( $args['site_id'] ) ) {
-		unset( $args['site_id'] );
+	$prepared_data = wp_prepare_site_data( $data, $defaults );
+	if ( is_wp_error( $prepared_data ) ) {
+		return $prepared_data;
 	}
 
-	$data = wp_prepare_site_data( $data, $defaults );
-	if ( is_wp_error( $data ) ) {
-		return $data;
-	}
-
-	if ( false === $wpdb->insert( $wpdb->blogs, $data ) ) {
+	if ( false === $wpdb->insert( $wpdb->blogs, $prepared_data ) ) {
 		return new WP_Error( 'db_insert_error', __( 'Could not insert site into the database.' ), $wpdb->last_error );
 	}
 
@@ -84,6 +78,12 @@ function wp_insert_site( array $data ) {
 	 */
 	do_action( 'wp_insert_site', $new_site );
 
+	// Extract the passed arguments that may be relevant for site initialization.
+	$args = array_diff_key( $data, $defaults );
+	if ( isset( $args['site_id'] ) ) {
+		unset( $args['site_id'] );
+	}
+
 	/**
 	 * Fires when a site's initialization routine should be executed.
 	 *
@@ -99,6 +99,16 @@ function wp_insert_site( array $data ) {
 		$user_id = ! empty( $args['user_id'] ) ? $args['user_id'] : 0;
 		$meta    = ! empty( $args['options'] ) ? $args['options'] : array();
 
+		// WPLANG was passed with `$meta` to the `wpmu_new_blog` hook prior to 5.1.0.
+		if ( ! array_key_exists( 'WPLANG', $meta ) ) {
+			$meta['WPLANG'] = get_network_option( $new_site->network_id, 'WPLANG' );
+		}
+
+		// Rebuild the data expected by the `wpmu_new_blog` hook prior to 5.1.0 using whitelisted keys.
+		// The `$site_data_whitelist` matches the one used in `wpmu_create_blog()`.
+		$site_data_whitelist = array( 'public', 'archived', 'mature', 'spam', 'deleted', 'lang_id' );
+		$meta = array_merge( array_intersect_key( $data, array_flip( $site_data_whitelist ) ), $meta );
+
 		/**
 		 * Fires immediately after a new site is created.
 		 *
diff --git a/tests/phpunit/tests/multisite/site.php b/tests/phpunit/tests/multisite/site.php
index 5397e85dcf..ff785c0a0a 100644
--- a/tests/phpunit/tests/multisite/site.php
+++ b/tests/phpunit/tests/multisite/site.php
@@ -12,6 +12,7 @@ if ( is_multisite() ) :
 		protected $suppress                = false;
 		protected $site_status_hooks       = array();
 		protected $wp_initialize_site_args = array();
+		protected $wp_initialize_site_meta = array();
 		protected static $network_ids;
 		protected static $site_ids;
 		protected static $uninitialized_site_id;
@@ -2348,6 +2349,84 @@ if ( is_multisite() ) :
 			// Set siteurl
 			update_option( 'siteurl', 'http://testsite1.example.org/test' );
 		}
+
+		/**
+		 * Tests whether all expected meta are provided in deprecated `wpmu_new_blog` action.
+		 *
+		 * @dataProvider data_wpmu_new_blog_action_backward_commpatible
+		 *
+		 * @ticket 46351
+		 */
+		public function test_wpmu_new_blog_action_backward_compatible( $meta, $expected_meta ) {
+			// We are testing deprecated hook. Register it to expected deprecated notices.
+			$this->setExpectedDeprecated( 'wpmu_new_blog' );
+			add_action( 'wpmu_new_blog', array( $this, 'wpmu_new_blog_callback' ), 10, 6 );
+
+			wpmu_create_blog( 'testsite1.example.org', '/new-blog/', 'New Blog', get_current_user_id(), $meta, 1 );
+
+			$this->assertEquals( $expected_meta, $this->wp_initialize_site_meta );
+
+			$this->wp_initialize_site_meta = array();
+		}
+
+		/**
+		 * Capture the $meta value passed to the wpmu_new_blog action and compare it.
+		 */
+		public function wpmu_new_blog_callback( $blog_id, $user_id, $domain, $path, $network_id, $meta ) {
+			$this->wp_initialize_site_meta = $meta;
+		}
+
+		public function data_wpmu_new_blog_action_backward_commpatible() {
+			return array(
+				'default values'  => array(
+					array(
+					),
+					array(
+						'public' => 0, // `public` is one of the defaults metas in `wpmu_create_blog' function prior WordPress 5.1.0
+						'WPLANG' => 'en_US', // WPLANG is another default meta in `wpmu_create_blog` function prior WordPress 5.1.0.
+					),
+				),
+				'public site'     => array(
+					array(
+						'public' => 1,
+					),
+					array(
+						'public' => 1,
+						'WPLANG' => 'en_US'
+					),
+				),
+				'all whitelisted' => array(
+					array(
+						'public'   => -1,
+						'archived' => 0,
+						'mature'   => 0,
+						'spam'     => 0,
+						'deleted'  => 0,
+						'lang_id'  => 11,
+
+					),
+					array(
+						'public'   => -1,
+						'WPLANG'   => 'en_US',
+						'archived' => 0,
+						'mature'   => 0,
+						'spam'     => 0,
+						'deleted'  => 0,
+						'lang_id'  => 11,
+					),
+				),
+				'extra meta key'  => array(
+					array(
+						'foo' => 'bar',
+					),
+					array(
+						'public' => 0,
+						'WPLANG' => 'en_US',
+						'foo'    => 'bar',
+					),
+				),
+			);
+		}
 	}
 
 endif;