diff --git a/src/wp-includes/l10n.php b/src/wp-includes/l10n.php
index e4acf40b38..bee3581618 100644
--- a/src/wp-includes/l10n.php
+++ b/src/wp-includes/l10n.php
@@ -1151,6 +1151,7 @@ function load_script_textdomain( $handle, $domain = 'default', $path = '' ) {
 	$content_url = wp_parse_url( content_url() );
 	$plugins_url = wp_parse_url( plugins_url() );
 	$site_url    = wp_parse_url( site_url() );
+	$theme_root  = get_theme_root();
 
 	// If the host is the same or it's a relative URL.
 	if (
@@ -1167,12 +1168,13 @@ function load_script_textdomain( $handle, $domain = 'default', $path = '' ) {
 		$relative = explode( '/', $relative );
 
 		/*
-		 * Ensure correct languages path when using a custom `WP_PLUGIN_DIR` / `WP_PLUGIN_URL` configuration.
+		 * Ensure correct languages path when using a custom `WP_PLUGIN_DIR` / `WP_PLUGIN_URL` configuration,
+		 * a custom theme root, and/or using Multisite with subdirectories.
 		 * See https://core.trac.wordpress.org/ticket/60891 and https://core.trac.wordpress.org/ticket/62016.
 		 */
-		$plugins_dir = array_slice( explode( '/', $plugins_url['path'] ), 2 );
-		$plugins_dir = trim( $plugins_dir[0], '/' );
-		$dirname     = $plugins_dir === $relative[0] ? 'plugins' : 'themes';
+
+		$theme_dir = array_slice( explode( '/', $theme_root ), -1 );
+		$dirname   = $theme_dir[0] === $relative[0] ? 'themes' : 'plugins';
 
 		$languages_path = WP_LANG_DIR . '/' . $dirname;
 
diff --git a/tests/phpunit/tests/l10n/loadScriptTextdomain.php b/tests/phpunit/tests/l10n/loadScriptTextdomain.php
index 1f5fde9eec..bc028fbfa5 100644
--- a/tests/phpunit/tests/l10n/loadScriptTextdomain.php
+++ b/tests/phpunit/tests/l10n/loadScriptTextdomain.php
@@ -28,7 +28,7 @@ class Tests_L10n_LoadScriptTextdomain extends WP_UnitTestCase {
 		$this->assertSame( $expected, load_script_textdomain( $handle, $textdomain, DIR_TESTDATA . '/languages' ) );
 	}
 
-	public function data_resolve_relative_path() {
+	public static function data_resolve_relative_path() {
 		return array(
 			// @ticket 45528
 			array(
@@ -43,7 +43,7 @@ class Tests_L10n_LoadScriptTextdomain extends WP_UnitTestCase {
 				'test-example-cdn',
 				'https://my-cdn.com/wordpress/wp-includes/js/script.js',
 				'default',
-				array( 'load_script_textdomain_relative_path', array( $this, 'relative_path_from_cdn' ), 2 ),
+				array( 'load_script_textdomain_relative_path', array( __CLASS__, 'relative_path_from_cdn' ), 2 ),
 			),
 			// Test for WordPress installs in a subdirectory.
 			array(
@@ -146,7 +146,7 @@ class Tests_L10n_LoadScriptTextdomain extends WP_UnitTestCase {
 		);
 	}
 
-	public function relative_path_from_cdn( $relative, $src ) {
+	public static function relative_path_from_cdn( $relative, $src ) {
 		if ( 0 === strpos( $src, 'https://my-cdn.com/wordpress/' ) ) {
 			return substr( $src, strlen( 'https://my-cdn.com/wordpress/' ) );
 		}