FEATURE: Also localize topic excerpts (#32839)

Currently, topic excerpts are not localized.

This commit adds the excerpt column into topic_localization and displays the
localized excerpt if present.

app/models/topic_localization.rb

@@ -22,6 +22,7 @@ end
 #  localizer_user_id :integer          not null
 #  created_at        :datetime         not null
 #  updated_at        :datetime         not null
+#  excerpt           :string
 #
 # Indexes
 #

app/serializers/listable_topic_serializer.rb

@@ -111,7 +111,13 @@ class ListableTopicSerializer < BasicTopicSerializer
   end
 
   def excerpt
-    object.excerpt
+    e = object.excerpt
+
+    if (ContentLocalization.show_translated_topic?(object, scope))
+      object.get_localization&.excerpt.presence || e
+    else
+      e
+    end
   end
 
   alias include_last_read_post_number? has_user_data

db/migrate/20250521053324_add_excerpt_to_topic_localization.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddExcerptToTopicLocalization < ActiveRecord::Migration[7.2]
+  def change
+    add_column :topic_localizations, :excerpt, :string, null: true, default: nil
+  end
+end

spec/serializers/listable_topic_serializer_spec.rb

@@ -1,9 +1,12 @@
 # frozen_string_literal: true
-RSpec.describe ListableTopicSerializer do
+
+describe ListableTopicSerializer do
   fab!(:topic)
 
   describe "#excerpt" do
-    it "can be extended by theme modifiers" do
+    before { topic.update!(excerpt: "This is excerrrpt-ional") }
+
+    it "can be included by theme modifiers" do
       payload = TopicListItemSerializer.new(topic, scope: Guardian.new, root: false).as_json
 
       expect(payload[:excerpt]).to eq(nil)
@@ -24,5 +27,31 @@ RSpec.describe ListableTopicSerializer do
 
       expect(payload[:excerpt]).to eq(topic.excerpt)
     end
+
+    it "does not include the excerpt by default" do
+      json = ListableTopicSerializer.new(topic, scope: Guardian.new).as_json
+
+      expect(json[:listable_topic][:excerpt]).to eq(nil)
+    end
+
+    it "returns the topic's excerpt" do
+      SiteSetting.always_include_topic_excerpts = true
+      json = ListableTopicSerializer.new(topic, scope: Guardian.new).as_json
+
+      expect(json[:listable_topic][:excerpt]).to eq("This is excerrrpt-ional")
+    end
+
+    it "returns the localized excerpt when setting is enabled" do
+      I18n.locale = "ja"
+      topic.update!(locale: "en")
+      Fabricate(:topic_localization, topic:, excerpt: "X", locale: "ja")
+
+      SiteSetting.experimental_content_localization = true
+      SiteSetting.always_include_topic_excerpts = true
+
+      json = ListableTopicSerializer.new(topic, scope: Guardian.new).as_json
+
+      expect(json[:listable_topic][:excerpt]).to eq("X")
+    end
   end
 end