Add unit test

tests/phpunit/data/plugins/custom-internationalized-plugin/custom-internationalized-plugin.php

@@ -7,7 +7,11 @@
 Text Domain: custom-internationalized-plugin
 */
 
-load_plugin_textdomain( 'custom-internationalized-plugin', false, dirname( plugin_basename( __FILE__ ) ) . '/languages' );
+function custom_i18n_load_textdomain() {
+	load_plugin_textdomain( 'custom-internationalized-plugin', false, dirname( plugin_basename( __FILE__ ) ) . '/languages' );
+}
+
+add_action( 'init', 'custom_i18n_load_textdomain' );
 
 function custom_i18n_plugin_test() {
 	return __( 'This is a dummy plugin', 'custom-internationalized-plugin' );

tests/phpunit/tests/l10n/loadTextdomainJustInTime.php

@@ -342,4 +342,36 @@ public function test_get_locale_is_called_only_once_per_textdomain_with_custom_l
 		$this->assertFalse( is_textdomain_loaded( $textdomain ) );
 		$this->assertSame( 1, $filter->get_call_count() );
 	}
+
+	/**
+	 * @ticket 44937
+	 * @ticket 62337
+	 *
+	 * @covers ::load_plugin_textdomain
+	 * @covers ::is_textdomain_loaded
+	 * @covers WP_Textdomain_Registry::set_custom_path
+	 */
+	public function test_plugin_translation_should_be_translated_when_calling_load_plugin_textdomain_too_late() {
+		require_once DIR_TESTDATA . '/plugins/custom-internationalized-plugin/custom-internationalized-plugin.php';
+
+		add_filter( 'locale', array( $this, 'filter_set_locale_to_german' ) );
+
+		$is_textdomain_loaded_before = is_textdomain_loaded( 'custom-internationalized-plugin' );
+		$output_before               = custom_i18n_plugin_test();
+
+		$is_textdomain_loaded_middle = is_textdomain_loaded( 'custom-internationalized-plugin' );
+
+		custom_i18n_load_textdomain();
+
+		$output_after               = custom_i18n_plugin_test();
+		$is_textdomain_loaded_after = is_textdomain_loaded( 'custom-internationalized-plugin' );
+
+		remove_filter( 'locale', array( $this, 'filter_set_locale_to_german' ) );
+
+		$this->assertFalse( $is_textdomain_loaded_before );
+		$this->assertFalse( $is_textdomain_loaded_middle );
+		$this->assertSame( 'This is a dummy plugin', $output_before );
+		$this->assertSame( 'Das ist ein Dummy Plugin', $output_after );
+		$this->assertTrue( $is_textdomain_loaded_after );
+	}
 }

tests/phpunit/tests/l10n/wpLocaleSwitcher.php

@@ -493,6 +493,8 @@ public function test_switch_reloads_plugin_translations_outside_wp_lang_dir() {
 
 		require_once DIR_TESTDATA . '/plugins/custom-internationalized-plugin/custom-internationalized-plugin.php';
 
+		custom_i18n_load_textdomain();
+
 		$actual = custom_i18n_plugin_test();
 
 		switch_to_locale( 'es_ES' );