diff --git a/app/components/companion_windows_component.html.erb b/app/components/companion_windows_component.html.erb
index d28d4fcea..cc0b53b88 100644
--- a/app/components/companion_windows_component.html.erb
+++ b/app/components/companion_windows_component.html.erb
@@ -43,7 +43,7 @@
 
         <% if render_content_list_panel? %>
           <section id="object-content" role="tabpanel">
-            <%= render ContentListComponent.new(viewer:, heading: viewer.content_heading) %>
+            <%= render ContentListComponent.new(viewer:) %>
           </section>
         <% end %>
 
diff --git a/app/components/content_list_component.rb b/app/components/content_list_component.rb
index fdf83ecaf..911baca80 100644
--- a/app/components/content_list_component.rb
+++ b/app/components/content_list_component.rb
@@ -1,10 +1,13 @@
 # frozen_string_literal: true
 
 class ContentListComponent < ViewComponent::Base
-  def initialize(viewer:, heading:)
+  def initialize(viewer:)
     @viewer = viewer
-    @heading = heading
   end
 
-  attr_reader :viewer, :heading
+  attr_reader :viewer
+
+  def heading
+    t('content_list_heading', scope: viewer.i18n_path)
+  end
 end
diff --git a/app/viewers/embed/viewer/common_viewer.rb b/app/viewers/embed/viewer/common_viewer.rb
index 761582895..3f7458f95 100644
--- a/app/viewers/embed/viewer/common_viewer.rb
+++ b/app/viewers/embed/viewer/common_viewer.rb
@@ -106,8 +106,12 @@ def display_header?
         !embed_request.hide_title?
       end
 
+      def i18n_path
+        "viewers.#{self.class.name.demodulize.underscore}"
+      end
+
       def iframe_title
-        I18n.t("viewers.#{self.class.name.demodulize.underscore}.title", default: 'Viewer')
+        I18n.t('title', default: 'Viewer', scope: i18n_path)
       end
 
       private
diff --git a/app/viewers/embed/viewer/media.rb b/app/viewers/embed/viewer/media.rb
index a0764212b..4a29113b9 100644
--- a/app/viewers/embed/viewer/media.rb
+++ b/app/viewers/embed/viewer/media.rb
@@ -11,11 +11,6 @@ def importmap
         'media'
       end
 
-      # The heading shown in the Content List of the CompanionWindowComponent
-      def content_heading
-        'Media content'
-      end
-
       private
 
       def default_height
diff --git a/app/viewers/embed/viewer/pdf_viewer.rb b/app/viewers/embed/viewer/pdf_viewer.rb
index c3f9990dc..c0c160def 100644
--- a/app/viewers/embed/viewer/pdf_viewer.rb
+++ b/app/viewers/embed/viewer/pdf_viewer.rb
@@ -38,11 +38,6 @@ def available?
         document_resource_files.first&.downloadable?
       end
 
-      # The heading shown in the Content List of the CompanionWindowComponent
-      def content_heading
-        'Contents'
-      end
-
       private
 
       def document_resource_files
diff --git a/config/environments/test.rb b/config/environments/test.rb
index 14bc29e06..c188a4da3 100644
--- a/config/environments/test.rb
+++ b/config/environments/test.rb
@@ -4,6 +4,7 @@
 # and recreated between test runs. Don't rely on the data there!
 
 Rails.application.configure do
+  config.i18n.raise_on_missing_translations = true
   # Settings specified here will take precedence over those in config/application.rb.
 
   # While tests run files are not watched, reloading is not necessary.
diff --git a/config/locales/en.yml b/config/locales/en.yml
index a0a0a0e78..dad1849a1 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -21,12 +21,12 @@
 
 en:
   restrictions:
-    not_accessible: 'This item cannot be accessed online. See Access conditions for more information.'
-    logged_in: 'Logged in.'
+    not_accessible: "This item cannot be accessed online. See Access conditions for more information."
+    logged_in: "Logged in."
   date:
     formats:
-      sul: '%d-%b-%Y'
-      sul_was_long: '%d %B %Y'
+      sul: "%d-%b-%Y"
+      sul_was_long: "%d %B %Y"
   number:
     human:
       storage_units:
@@ -34,20 +34,22 @@ en:
           kb: kB # This is the SI unit for 10^3
   viewers:
     file:
-      title: 'File viewer'
+      title: "File viewer"
     geo:
-      title: 'Geospatial viewer'
+      title: "Geospatial viewer"
     m3_viewer:
-      title: 'Image viewer'
+      title: "Image viewer"
     media:
-      title: 'Media viewer'
+      title: "Media viewer"
+      content_list_heading: Media content
     pdf_viewer:
-      title: 'PDF viewer'
+      title: "PDF viewer"
+      content_list_heading: Contents
     model_viewer:
-      title: '3-D object viewer'
+      title: "3-D object viewer"
     was_seed:
-      title: 'File viewer'
+      title: "File viewer"
   download_files:
-    zero: 'No file available for download'
-    one: '1 file available for download'
-    other: '%{count} files available for download'
+    zero: "No file available for download"
+    one: "1 file available for download"
+    other: "%{count} files available for download"