diff --git a/wp-includes/class-wp-locale-switcher.php b/wp-includes/class-wp-locale-switcher.php
index 7ec7622161..8a9bbc3903 100644
--- a/wp-includes/class-wp-locale-switcher.php
+++ b/wp-includes/class-wp-locale-switcher.php
@@ -196,11 +196,12 @@ class WP_Locale_Switcher {
 		load_default_textdomain( $locale );
 
 		foreach ( $domains as $domain ) {
+			// The default text domain is handled by `load_default_textdomain()`.
 			if ( 'default' === $domain ) {
 				continue;
 			}
 
-			unload_textdomain( $domain );
+			unload_textdomain( $domain, true );
 			get_translations_for_domain( $domain );
 		}
 	}
@@ -218,12 +219,11 @@ class WP_Locale_Switcher {
 	 * @param string $locale The locale to change to.
 	 */
 	private function change_locale( $locale ) {
-		// Reset translation availability information.
-		_get_path_to_translation( null, true );
+		global $wp_locale;
 
 		$this->load_translations( $locale );
 
-		$GLOBALS['wp_locale'] = new WP_Locale();
+		$wp_locale = new WP_Locale();
 
 		/**
 		 * Fires when the locale is switched to or restored.
diff --git a/wp-includes/class-wp-textdomain-registry.php b/wp-includes/class-wp-textdomain-registry.php
new file mode 100644
index 0000000000..5a5da5ee9c
--- /dev/null
+++ b/wp-includes/class-wp-textdomain-registry.php
@@ -0,0 +1,130 @@
+<?php
+/**
+ * Locale API: WP_Textdomain_Registry class
+ *
+ * @package WordPress
+ * @subpackage i18n
+ * @since 5.6.0
+ */
+
+/**
+ * Core class used for registering text domains.
+ *
+ * @since 5.6.0
+ */
+class WP_Textdomain_Registry {
+	/**
+	 * List of domains and their language directory paths.
+	 *
+	 * @since 5.6.0
+	 *
+	 * @var array
+	 */
+	protected $domains = array();
+
+	/**
+	 * Holds a cached list of available .mo files to improve performance.
+	 *
+	 * @since 5.6.0
+	 *
+	 * @var array
+	 */
+	protected $cached_mo_files;
+
+	/**
+	 * Returns the MO file path for a specific domain.
+	 *
+	 * @since 5.6.0
+	 *
+	 * @param string $domain Text domain.
+	 * @return string|false MO file path or false if there is none available.
+	 */
+	public function get( $domain ) {
+		if ( isset( $this->domains[ $domain ] ) ) {
+			return $this->domains[ $domain ];
+		}
+
+		return $this->get_path_from_lang_dir( $domain );
+	}
+
+	/**
+	 * Sets the MO file path for a specific domain.
+	 *
+	 * @since 5.6.0
+	 *
+	 * @param string $domain Text domain.
+	 * @param string|false $path Language directory path or false if there is none available.
+	 */
+	public function set( $domain, $path ) {
+		$this->domains[ $domain ] = $path ? trailingslashit( $path ) : false;
+	}
+
+	/**
+	 * Resets the registry state.
+	 *
+	 * @since 5.6.0
+	 */
+	public function reset() {
+		$this->cached_mo_files = null;
+		$this->domains         = array();
+	}
+
+	/**
+	 * Gets the path to a translation file in the languages directory for the current locale.
+	 *
+	 * @since 5.6.0
+	 *
+	 * @param string $domain Text domain.
+	 * @return string|false MO file path or false if there is none available.
+	 */
+	private function get_path_from_lang_dir( $domain ) {
+		if ( null === $this->cached_mo_files ) {
+			$this->cached_mo_files = array();
+
+			$this->set_cached_mo_files();
+		}
+
+		$locale = determine_locale();
+		$mofile = "{$domain}-{$locale}.mo";
+
+		$path = WP_LANG_DIR . '/plugins/' . $mofile;
+		if ( in_array( $path, $this->cached_mo_files, true ) ) {
+			$path = WP_LANG_DIR . '/plugins/';
+			$this->set( $domain, $path );
+
+			return $path;
+		}
+
+		$path = WP_LANG_DIR . '/themes/' . $mofile;
+		if ( in_array( $path, $this->cached_mo_files, true ) ) {
+			$path = WP_LANG_DIR . '/themes/';
+			$this->set( $domain, $path );
+
+			return $path;
+		}
+
+		$this->set( $domain, false );
+
+		return false;
+	}
+
+	/**
+	 * Reads and caches all available MO files from the plugins and themes language directories.
+	 *
+	 * @since 5.6.0
+	 */
+	protected function set_cached_mo_files() {
+		$locations = array(
+			WP_LANG_DIR . '/plugins',
+			WP_LANG_DIR . '/themes',
+		);
+
+		foreach ( $locations as $location ) {
+			$mo_files = glob( $location . '/*.mo' );
+
+			if ( $mo_files ) {
+				$this->cached_mo_files = array_merge( $this->cached_mo_files, $mo_files );
+			}
+		}
+	}
+}
diff --git a/wp-includes/deprecated.php b/wp-includes/deprecated.php
index 44999504dc..ca83d3dfb0 100644
--- a/wp-includes/deprecated.php
+++ b/wp-includes/deprecated.php
@@ -4134,3 +4134,85 @@ function wp_slash_strings_only( $value ) {
 function addslashes_strings_only( $value ) {
 	return is_string( $value ) ? addslashes( $value ) : $value;
 }
+
+/**
+ * Gets the path to a translation file for loading a textdomain just in time.
+ *
+ * Caches the retrieved results internally.
+ *
+ * @since 4.7.0
+ * @deprecated 5.6.0
+ * @access private
+ *
+ * @see _load_textdomain_just_in_time()
+ *
+ * @param string $domain Text domain. Unique identifier for retrieving translated strings.
+ * @param bool   $reset  Whether to reset the internal cache. Used by the switch to locale functionality.
+ * @return string|false The path to the translation file or false if no translation file was found.
+ */
+function _get_path_to_translation( $domain, $reset = false ) {
+	_deprecated_function( __FUNCTION__, '5.6.0', 'WP_Textdomain_Registry' );
+
+	static $available_translations = array();
+
+	if ( true === $reset ) {
+		$available_translations = array();
+	}
+
+	if ( ! isset( $available_translations[ $domain ] ) ) {
+		$available_translations[ $domain ] = _get_path_to_translation_from_lang_dir( $domain );
+	}
+
+	return $available_translations[ $domain ];
+}
+
+/**
+ * Gets the path to a translation file in the languages directory for the current locale.
+ *
+ * Holds a cached list of available .mo files to improve performance.
+ *
+ * @since 4.7.0
+ * @deprecated 5.6.0
+ * @access private
+ *
+ * @see _get_path_to_translation()
+ *
+ * @param string $domain Text domain. Unique identifier for retrieving translated strings.
+ * @return string|false The path to the translation file or false if no translation file was found.
+ */
+function _get_path_to_translation_from_lang_dir( $domain ) {
+	_deprecated_function( __FUNCTION__, '5.6.0', 'WP_Textdomain_Registry' );
+
+	static $cached_mofiles = null;
+
+	if ( null === $cached_mofiles ) {
+		$cached_mofiles = array();
+
+		$locations = array(
+			WP_LANG_DIR . '/plugins',
+			WP_LANG_DIR . '/themes',
+		);
+
+		foreach ( $locations as $location ) {
+			$mofiles = glob( $location . '/*.mo' );
+			if ( $mofiles ) {
+				$cached_mofiles = array_merge( $cached_mofiles, $mofiles );
+			}
+		}
+	}
+
+	$locale = determine_locale();
+	$mofile = "{$domain}-{$locale}.mo";
+
+	$path = WP_LANG_DIR . '/plugins/' . $mofile;
+	if ( in_array( $path, $cached_mofiles, true ) ) {
+		return $path;
+	}
+
+	$path = WP_LANG_DIR . '/themes/' . $mofile;
+	if ( in_array( $path, $cached_mofiles, true ) ) {
+		return $path;
+	}
+
+	return false;
+}
diff --git a/wp-includes/l10n.php b/wp-includes/l10n.php
index 0c6b75ea24..6bca550e98 100644
--- a/wp-includes/l10n.php
+++ b/wp-includes/l10n.php
@@ -689,15 +689,16 @@ function translate_nooped_plural( $nooped_plural, $count, $domain = 'default' )
  *
  * @since 1.5.0
  *
- * @global MO[] $l10n          An array of all currently loaded text domains.
- * @global MO[] $l10n_unloaded An array of all text domains that have been unloaded again.
+ * @global MO[]                   $l10n                   An array of all currently loaded text domains.
+ * @global MO[]                   $l10n_unloaded          An array of all text domains that have been unloaded again.
+ * @global WP_Textdomain_Registry $wp_textdomain_registry WordPress Textdomain Registry.
  *
  * @param string $domain Text domain. Unique identifier for retrieving translated strings.
  * @param string $mofile Path to the .mo file.
  * @return bool True on success, false on failure.
  */
 function load_textdomain( $domain, $mofile ) {
-	global $l10n, $l10n_unloaded;
+	global $l10n, $l10n_unloaded, $wp_textdomain_registry;
 
 	$l10n_unloaded = (array) $l10n_unloaded;
 
@@ -755,6 +756,9 @@ function load_textdomain( $domain, $mofile ) {
 
 	$l10n[ $domain ] = &$mo;
 
+	/** @var WP_Textdomain_Registry $wp_textdomain_registry */
+	$wp_textdomain_registry->set( $domain, dirname( $mofile ) );
+
 	return true;
 }
 
@@ -762,14 +766,16 @@ function load_textdomain( $domain, $mofile ) {
  * Unload translations for a text domain.
  *
  * @since 3.0.0
+ * @since 5.6.0 Added the `$reloadable` parameter.
  *
  * @global MO[] $l10n          An array of all currently loaded text domains.
  * @global MO[] $l10n_unloaded An array of all text domains that have been unloaded again.
  *
- * @param string $domain Text domain. Unique identifier for retrieving translated strings.
+ * @param string $domain     Text domain. Unique identifier for retrieving translated strings.
+ * @param bool   $reloadable Whether the text domain can be loaded just-in-time again.
  * @return bool Whether textdomain was unloaded.
  */
-function unload_textdomain( $domain ) {
+function unload_textdomain( $domain, $reloadable = false ) {
 	global $l10n, $l10n_unloaded;
 
 	$l10n_unloaded = (array) $l10n_unloaded;
@@ -778,14 +784,18 @@ function unload_textdomain( $domain ) {
 	 * Filters whether to override the text domain unloading.
 	 *
 	 * @since 3.0.0
+	 * @since 5.6.0 Added the `$reloadable` parameter.
 	 *
-	 * @param bool   $override Whether to override the text domain unloading. Default false.
-	 * @param string $domain   Text domain. Unique identifier for retrieving translated strings.
+	 * @param bool   $override   Whether to override the text domain unloading. Default false.
+	 * @param string $domain     Text domain. Unique identifier for retrieving translated strings.
+	 * @param bool   $reloadable Whether the text domain can be loaded just-in-time again.
 	 */
-	$plugin_override = apply_filters( 'override_unload_textdomain', false, $domain );
+	$plugin_override = apply_filters( 'override_unload_textdomain', false, $domain, $reloadable );
 
 	if ( $plugin_override ) {
-		$l10n_unloaded[ $domain ] = true;
+		if ( ! $reloadable ) {
+			$l10n_unloaded[ $domain ] = true;
+		}
 
 		return true;
 	}
@@ -794,15 +804,19 @@ function unload_textdomain( $domain ) {
 	 * Fires before the text domain is unloaded.
 	 *
 	 * @since 3.0.0
+	 * @since 5.6.0 Added the `$reloadable` parameter.
 	 *
-	 * @param string $domain Text domain. Unique identifier for retrieving translated strings.
+	 * @param string $domain     Text domain. Unique identifier for retrieving translated strings.
+	 * @param bool   $reloadable Whether the text domain can be loaded just-in-time again.
 	 */
-	do_action( 'unload_textdomain', $domain );
+	do_action( 'unload_textdomain', $domain, $reloadable );
 
 	if ( isset( $l10n[ $domain ] ) ) {
 		unset( $l10n[ $domain ] );
 
-		$l10n_unloaded[ $domain ] = true;
+		if ( ! $reloadable ) {
+			$l10n_unloaded[ $domain ] = true;
+		}
 
 		return true;
 	}
@@ -867,6 +881,8 @@ function load_default_textdomain( $locale = null ) {
  * @return bool True when textdomain is successfully loaded, false otherwise.
  */
 function load_plugin_textdomain( $domain, $deprecated = false, $plugin_rel_path = false ) {
+	global $wp_textdomain_registry;
+
 	/**
 	 * Filters a plugin's locale.
 	 *
@@ -893,6 +909,9 @@ function load_plugin_textdomain( $domain, $deprecated = false, $plugin_rel_path
 		$path = WP_PLUGIN_DIR;
 	}
 
+	/* @var WP_Textdomain_Registry $wp_textdomain_registry */
+	$wp_textdomain_registry->set( $domain, $path );
+
 	return load_textdomain( $domain, $path . '/' . $mofile );
 }
 
@@ -902,12 +921,16 @@ function load_plugin_textdomain( $domain, $deprecated = false, $plugin_rel_path
  * @since 3.0.0
  * @since 4.6.0 The function now tries to load the .mo file from the languages directory first.
  *
+ * @global WP_Textdomain_Registry $wp_textdomain_registry WordPress Textdomain Registry.
+ *
  * @param string $domain             Text domain. Unique identifier for retrieving translated strings.
  * @param string $mu_plugin_rel_path Optional. Relative to `WPMU_PLUGIN_DIR` directory in which the .mo
  *                                   file resides. Default empty string.
  * @return bool True when textdomain is successfully loaded, false otherwise.
  */
 function load_muplugin_textdomain( $domain, $mu_plugin_rel_path = '' ) {
+	global $wp_textdomain_registry;
+
 	/** This filter is documented in wp-includes/l10n.php */
 	$locale = apply_filters( 'plugin_locale', determine_locale(), $domain );
 
@@ -920,6 +943,9 @@ function load_muplugin_textdomain( $domain, $mu_plugin_rel_path = '' ) {
 
 	$path = WPMU_PLUGIN_DIR . '/' . ltrim( $mu_plugin_rel_path, '/' );
 
+	/* @var WP_Textdomain_Registry $wp_textdomain_registry */
+	$wp_textdomain_registry->set( $domain, $path );
+
 	return load_textdomain( $domain, $path . '/' . $mofile );
 }
 
@@ -934,12 +960,16 @@ function load_muplugin_textdomain( $domain, $mu_plugin_rel_path = '' ) {
  * @since 1.5.0
  * @since 4.6.0 The function now tries to load the .mo file from the languages directory first.
  *
+ * @global WP_Textdomain_Registry $wp_textdomain_registry WordPress Textdomain Registry.
+ *
  * @param string $domain Text domain. Unique identifier for retrieving translated strings.
  * @param string $path   Optional. Path to the directory containing the .mo file.
  *                       Default false.
  * @return bool True when textdomain is successfully loaded, false otherwise.
  */
 function load_theme_textdomain( $domain, $path = false ) {
+	global $wp_textdomain_registry;
+
 	/**
 	 * Filters a theme's locale.
 	 *
@@ -961,6 +991,9 @@ function load_theme_textdomain( $domain, $path = false ) {
 		$path = get_template_directory();
 	}
 
+	/* @var WP_Textdomain_Registry $wp_textdomain_registry */
+	$wp_textdomain_registry->set( $domain, $path );
+
 	return load_textdomain( $domain, $path . '/' . $locale . '.mo' );
 }
 
@@ -1190,14 +1223,14 @@ function load_script_translations( $file, $handle, $domain ) {
  * @since 4.6.0
  * @access private
  *
- * @see get_translations_for_domain()
- * @global MO[] $l10n_unloaded An array of all text domains that have been unloaded again.
+ * @global MO[]                   $l10n_unloaded          An array of all text domains that have been unloaded again.
+ * @global WP_Textdomain_Registry $wp_textdomain_registry WordPress Textdomain Registry.
  *
  * @param string $domain Text domain. Unique identifier for retrieving translated strings.
  * @return bool True when the textdomain is successfully loaded, false otherwise.
  */
 function _load_textdomain_just_in_time( $domain ) {
-	global $l10n_unloaded;
+	global $l10n_unloaded, $wp_textdomain_registry;
 
 	$l10n_unloaded = (array) $l10n_unloaded;
 
@@ -1206,88 +1239,24 @@ function _load_textdomain_just_in_time( $domain ) {
 		return false;
 	}
 
-	$translation_path = _get_path_to_translation( $domain );
-	if ( false === $translation_path ) {
+	/** @var WP_Textdomain_Registry $wp_textdomain_registry */
+	$path = $wp_textdomain_registry->get( $domain );
+	if ( ! $path ) {
 		return false;
 	}
 
-	return load_textdomain( $domain, $translation_path );
-}
-
-/**
- * Gets the path to a translation file for loading a textdomain just in time.
- *
- * Caches the retrieved results internally.
- *
- * @since 4.7.0
- * @access private
- *
- * @see _load_textdomain_just_in_time()
- *
- * @param string $domain Text domain. Unique identifier for retrieving translated strings.
- * @param bool   $reset  Whether to reset the internal cache. Used by the switch to locale functionality.
- * @return string|false The path to the translation file or false if no translation file was found.
- */
-function _get_path_to_translation( $domain, $reset = false ) {
-	static $available_translations = array();
-
-	if ( true === $reset ) {
-		$available_translations = array();
-	}
-
-	if ( ! isset( $available_translations[ $domain ] ) ) {
-		$available_translations[ $domain ] = _get_path_to_translation_from_lang_dir( $domain );
-	}
-
-	return $available_translations[ $domain ];
-}
-
-/**
- * Gets the path to a translation file in the languages directory for the current locale.
- *
- * Holds a cached list of available .mo files to improve performance.
- *
- * @since 4.7.0
- * @access private
- *
- * @see _get_path_to_translation()
- *
- * @param string $domain Text domain. Unique identifier for retrieving translated strings.
- * @return string|false The path to the translation file or false if no translation file was found.
- */
-function _get_path_to_translation_from_lang_dir( $domain ) {
-	static $cached_mofiles = null;
-
-	if ( null === $cached_mofiles ) {
-		$cached_mofiles = array();
-
-		$locations = array(
-			WP_LANG_DIR . '/plugins',
-			WP_LANG_DIR . '/themes',
-		);
-
-		foreach ( $locations as $location ) {
-			$mofiles = glob( $location . '/*.mo' );
-			if ( $mofiles ) {
-				$cached_mofiles = array_merge( $cached_mofiles, $mofiles );
-			}
-		}
-	}
-
 	$locale = determine_locale();
-	$mofile = "{$domain}-{$locale}.mo";
 
-	$path = WP_LANG_DIR . '/plugins/' . $mofile;
-	if ( in_array( $path, $cached_mofiles, true ) ) {
-		return $path;
+	// Themes with their language directory outside of WP_LANG_DIR have a different file name.
+	$template_directory   = trailingslashit( get_template_directory() );
+	$stylesheet_directory = trailingslashit( get_stylesheet_directory() );
+	if ( 0 === strpos( $path, $template_directory ) || 0 === strpos( $path, $stylesheet_directory ) ) {
+		$mofile = "{$path}{$locale}.mo";
+	} else {
+		$mofile = "{$path}{$domain}-{$locale}.mo";
 	}
 
-	$path = WP_LANG_DIR . '/themes/' . $mofile;
-	if ( in_array( $path, $cached_mofiles, true ) ) {
-		return $path;
-	}
-
-	return false;
+	return load_textdomain( $domain, $mofile );
 }
 
 /**
@@ -1297,7 +1266,7 @@ function _get_path_to_translation_from_lang_dir( $domain ) {
  *
  * @since 2.8.0
  *
- * @global MO[] $l10n
+ * @global MO[] $l10n An array of all currently loaded text domains.
  *
  * @param string $domain Text domain. Unique identifier for retrieving translated strings.
  * @return Translations|NOOP_Translations A Translations instance.
@@ -1321,7 +1290,7 @@ function get_translations_for_domain( $domain ) {
  *
  * @since 3.0.0
  *
- * @global MO[] $l10n
+ * @global MO[] $l10n An array of all currently loaded text domains.
  *
  * @param string $domain Text domain. Unique identifier for retrieving translated strings.
  * @return bool Whether there are translations.
diff --git a/wp-includes/version.php b/wp-includes/version.php
index f79dc901f6..878b8b17b7 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.6-alpha-49235';
+$wp_version = '5.6-alpha-49236';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.
diff --git a/wp-settings.php b/wp-settings.php
index 71868ecc89..9f1f24beb0 100644
--- a/wp-settings.php
+++ b/wp-settings.php
@@ -152,6 +152,7 @@ if ( SHORTINIT ) {
 
 // Load the L10n library.
 require_once ABSPATH . WPINC . '/l10n.php';
+require_once ABSPATH . WPINC . '/class-wp-textdomain-registry.php';
 require_once ABSPATH . WPINC . '/class-wp-locale.php';
 require_once ABSPATH . WPINC . '/class-wp-locale-switcher.php';
 
@@ -301,6 +302,17 @@ require ABSPATH . WPINC . '/block-supports/typography.php';
 
 $GLOBALS['wp_embed'] = new WP_Embed();
 
+/**
+ * WordPress Textdomain Registry object.
+ *
+ * Used to support just-in-time translations for manually loaded textdomains.
+ *
+ * @since 5.6.0
+ *
+ * @global WP_Locale_Switcher $wp_locale_switcher WordPress Textdomain Registry.
+ */
+$GLOBALS['wp_textdomain_registry'] = new WP_Textdomain_Registry();
+
 // Load multisite-specific files.
 if ( is_multisite() ) {
 	require ABSPATH . WPINC . '/ms-functions.php';