DEV: Add locale for topics (#32619)

Similar to https://github.com/discourse/discourse/pull/32526.

Reviewer note: There had been some deliberation if we should just take
the `topic.first_post.locale` instead of having a `topic.locale`.
Ultimately, titles may be of a different language of the post itself,
and secondarily in some cases it could be faster without having to load
the first_post at all.

app/models/topic.rb

@@ -2233,6 +2233,7 @@ end
 #  bannered_until            :datetime
 #  external_id               :string
 #  visibility_reason_id      :integer
+#  locale                    :string(20)
 #
 # Indexes
 #

db/migrate/20250507110205_add_locale_to_topic.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddLocaleToTopic < ActiveRecord::Migration[7.2]
+  def change
+    add_column :topics, :locale, :string, limit: 20
+  end
+end