diff --git a/config_translation.module b/config_translation.module
index 008e80cad629104d71cabd66bcd0def91891b8fd..e3d20b8a20e40cdcf717c10df7e2f427c7d7c509 100644
--- a/config_translation.module
+++ b/config_translation.module
@@ -119,17 +119,19 @@ function config_translation_menu() {
 function config_translation_config_name_access(ConfigMapperInterface $mapper, $path_arg, $language = NULL) {
   // Get configuration group for this mapper.
   $group = $mapper->getConfigGroup($path_arg);
+  $group_language = language_load($group->getLangcode());
 
   // Only allow access to translate configuration, if proper permissions are
   // granted, the configuration has translatable pieces, the source language
-  // is known and the target language is not the original submission language.
-  // Although technically config can be overlayed with translations in the same
-  // language, that is logically not a good idea.
+  // and target language are not locked, and the target language is not the
+  // original submission language. Although technically config can be overlayed
+  // with translations in the same language, that is logically not a good idea.
   return (
     user_access('translate configuration') &&
     $group->hasSchema() &&
     $group->hasTranslatable() &&
-    (empty($language) || ($language->langcode != LANGUAGE_NOT_SPECIFIED && $language->langcode != $group->getLangcode()))
+    !$group_language->locked &&
+    (empty($language) || (!$language->locked && $language->langcode != $group_language->langcode))
   );
 }
 
diff --git a/lib/Drupal/config_translation/Tests/ConfigTranslationUITest.php b/lib/Drupal/config_translation/Tests/ConfigTranslationUITest.php
index 119a81d6ea3f77f1bc013f9a6324115190c1adfa..1840763fc085877ca7426e9caddc01c03eecf79c 100644
--- a/lib/Drupal/config_translation/Tests/ConfigTranslationUITest.php
+++ b/lib/Drupal/config_translation/Tests/ConfigTranslationUITest.php
@@ -247,6 +247,50 @@ class ConfigTranslationUITest extends WebTestBase {
     }
   }
 
+  /**
+   * Test source and target language edge cases.
+   */
+  function testSourceAndTargetLanguage() {
+    // Loading translation page for not-specified language (und)
+    // should return 403.
+    $this->drupalGet('admin/config/system/site-information/translate/add/und');
+    $this->assertResponse(403);
+
+    // Check the source language doesn't have 'Add' or 'Delete' link and
+    // make sure source language edit goes to original configuration page
+    // not the translation specific edit page.
+    $this->drupalGet('admin/config/system/site-information/translate');
+    $this->assertNoLinkByHref('admin/config/system/site-information/translate/edit/en');
+    $this->assertNoLinkByHref('admin/config/system/site-information/translate/add/en');
+    $this->assertNoLinkByHref('admin/config/system/site-information/translate/delete/en');
+    $this->assertLinkByHref('admin/config/system/site-information');
+
+    // Translation addition to source language should return 403.
+    $this->drupalGet('admin/config/system/site-information/translate/add/en');
+    $this->assertResponse(403);
+
+    // Translation editing in source language should return 403.
+    $this->drupalGet('admin/config/system/site-information/translate/edit/en');
+    $this->assertResponse(403);
+
+    // Translation deletion in source language should return 403.
+    $this->drupalGet('admin/config/system/site-information/translate/delete/en');
+    $this->assertResponse(403);
+
+    // Set default language of site information to not-specified language (und).
+    config('system.site')
+      ->set('langcode', LANGUAGE_NOT_SPECIFIED)
+      ->save();
+
+    // Make sure translation tab does not exist on the config page.
+    $this->drupalGet('admin/config/system/site-information');
+    $this->assertNoLinkByHref('admin/config/system/site-information/translate');
+
+    // If source language is not specified, translation page should be 403.
+    $this->drupalGet('admin/config/system/site-information/translate');
+    $this->assertResponse(403);
+  }
+
   /**
    * Helper to set site name and slogan for default language.
    *