Issue #452244 by jhedstrom: Help text for Language domain needs to specify "no trailing slash"

core/modules/language/src/Form/NegotiationUrlForm.php

@@ -188,13 +188,13 @@ public function validateForm(array &$form, FormStateInterface $form_state) {
     }
 
     // Domain names should not contain protocol and/or ports.
-    foreach ($languages as $langcode => $name) {
+    foreach ($languages as $langcode => $language) {
       $value = $form_state->getValue(array('domain', $langcode));
       if (!empty($value)) {
         // Ensure we have exactly one protocol when checking the hostname.
         $host = 'http://' . str_replace(array('http://', 'https://'), '', $value);
         if (parse_url($host, PHP_URL_HOST) != $value) {
-          $form_state->setErrorByName("domain][$langcode", $this->t('The domain for %language may only contain the domain name, not a protocol and/or port.', array('%language' => $name)));
+          $form_state->setErrorByName("domain][$langcode", $this->t('The domain for %language may only contain the domain name, not a trailing slash, protocol and/or port.', ['%language' => $language->getName()]));
         }
       }
     }

core/modules/language/src/Tests/LanguageUILanguageNegotiationTest.php

@@ -450,6 +450,15 @@ function testLanguageDomain() {
     $this->assertText('The configuration options have been saved', 'Domain configuration is saved.');
     $this->rebuildContainer();
 
+    // Try to use an invalid domain.
+    $edit = [
+      'language_negotiation_url_part' => LanguageNegotiationUrl::CONFIG_DOMAIN,
+      'domain[en]' => $base_url_host,
+      'domain[it]' => 'it.example.com/',
+    ];
+    $this->drupalPostForm('admin/config/regional/language/detection/url', $edit, t('Save configuration'));
+    $this->assertRaw(t('The domain for %language may only contain the domain name, not a trailing slash, protocol and/or port.', ['%language' => 'Italian']));
+
     // Build the link we're going to test.
     $link = 'it.example.com' . rtrim(base_path(), '/') . '/admin';