FEATURE: Add locale as a filter on topic /filter (#35299)

Adds `locale` as a new filter on /filter for topics.

- locale:en -- single locale filtering
- locale:ja,es -- comma-separated OR logic
- locale:ja locale:es -- space-separated OR logic
- -locale:en -- exclusion of single locale
- -locale:en,ja -- exclusion of multiple locales
- locale:en status:closed -- combination with other filters

config/locales/server.en.yml

@@ -2952,6 +2952,9 @@ en:
       group: "Filter topics by groups"
       groups_any: "Show topics with any of the specified groups (comma-separated)"
       groups_all: "Show topics with all of the specified groups (plus-separated)"
+      locale: "Filter topics by original language"
+      locale_any: "Show topics originally written in any of the specified languages (comma-separated)"
+      exclude_locale: "Exclude topics originally written in a specific language"
 
   discourse_connect:
     login_error: "Login Error"

lib/topics_filter.rb

@@ -87,6 +87,8 @@ class TopicsFilter
         filter_tag_groups(values: key_prefixes.zip(filter_values))
       when "tag"
         filter_tags(values: key_prefixes.zip(filter_values))
+      when "locale"
+        filter_locale(values: key_prefixes.zip(filter_values))
       when "views-min"
         filter_by_number_of_views(min: filter_values)
       when "views-max"
@@ -343,6 +345,17 @@ class TopicsFilter
       },
     )
 
+    # Locale filter
+    results.push(
+      {
+        name: "locale:",
+        description: I18n.t("filter.description.locale"),
+        type: "text",
+        delimiters: [{ name: ",", description: I18n.t("filter.description.locale_any") }],
+        prefixes: [{ name: "-", description: I18n.t("filter.description.exclude_locale") }],
+      },
+    )
+
     # this modifier allows custom plugins to add UI tips in the /filter route
     DiscoursePluginRegistry.apply_modifier(:topics_filter_options, results, guardian)
   end
@@ -902,6 +915,28 @@ class TopicsFilter
         .distinct(:id)
   end
 
+  def filter_locale(values:)
+    include_locales = []
+    exclude_locales = []
+
+    values.each do |key_prefix, value|
+      locales = value.split(",").map(&:strip).reject(&:blank?)
+      next if locales.empty?
+
+      if key_prefix == "-"
+        exclude_locales.concat(locales)
+      else
+        include_locales.concat(locales)
+      end
+    end
+
+    @scope = @scope.where(locale: include_locales) if include_locales.present?
+
+    if exclude_locales.present?
+      @scope = @scope.where("topics.locale IS NULL OR topics.locale NOT IN (?)", exclude_locales)
+    end
+  end
+
   ORDER_BY_MAPPINGS = {
     "activity" => {
       column: "topics.bumped_at",

spec/lib/topics_filter_spec.rb

@@ -1419,6 +1419,93 @@ RSpec.describe TopicsFilter do
         ).to eq([])
       end
     end
+
+    describe "when filtering by locale" do
+      fab!(:en_topic) { Fabricate(:topic, locale: "en") }
+      fab!(:ja_topic) { Fabricate(:topic, locale: "ja") }
+      fab!(:es_topic) { Fabricate(:topic, locale: "es") }
+      fab!(:no_locale_topic, :topic)
+
+      describe "when query string is `locale:en`" do
+        it "should only return topics with locale en" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("locale:en")
+              .pluck(:id),
+          ).to contain_exactly(en_topic.id)
+        end
+      end
+
+      describe "when query string is `locale:ja,es`" do
+        it "should return topics with locale ja or es" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("locale:ja,es")
+              .pluck(:id),
+          ).to contain_exactly(ja_topic.id, es_topic.id)
+        end
+      end
+
+      describe "when query string is `locale:ja locale:es`" do
+        it "should return topics with locale ja or es" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("locale:ja locale:es")
+              .pluck(:id),
+          ).to contain_exactly(ja_topic.id, es_topic.id)
+        end
+      end
+
+      describe "when query string is `-locale:en`" do
+        it "should return topics without locale en" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("-locale:en")
+              .pluck(:id),
+          ).to contain_exactly(ja_topic.id, es_topic.id, no_locale_topic.id)
+        end
+      end
+
+      describe "when query string is `-locale:en,ja`" do
+        it "should return topics without locale en or ja" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("-locale:en,ja")
+              .pluck(:id),
+          ).to contain_exactly(es_topic.id, no_locale_topic.id)
+        end
+      end
+
+      describe "when query string is `locale:invalid`" do
+        it "should return no topics" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("locale:invalid")
+              .pluck(:id),
+          ).to eq([])
+        end
+      end
+
+      describe "when combining with other filters" do
+        before { en_topic.update!(closed: true) }
+
+        it "should work with status:closed" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("locale:en status:closed")
+              .pluck(:id),
+          ).to contain_exactly(en_topic.id)
+        end
+      end
+    end
+
     describe "when filtering by topic author" do
       fab!(:user2) { Fabricate(:user, username: "username2") }
       fab!(:topic_by_user) { Fabricate(:topic, user: user) }