diff --git a/.discourse-compatibility b/.discourse-compatibility
index 73df02b1..1e41b6e8 100644
--- a/.discourse-compatibility
+++ b/.discourse-compatibility
@@ -1,3 +1,4 @@
+< 3.5.0.beta6-dev: 3e74eea1e5e3143888d67a8d8a11206df214dc24
 < 3.5.0.beta3-dev: 09a68414804a1447f52e5d60691ba59742cda9ec
 < 3.5.0.beta2-dev: de8624416a15b3d8e7ad350b083cc1420451ccec
 < 3.5.0.beta1-dev: bdef136080074a993e7c4f5ca562edc31a8ba756
diff --git a/app/jobs/regular/localize_categories.rb b/app/jobs/regular/localize_categories.rb
index f123fd52..f370e2e7 100644
--- a/app/jobs/regular/localize_categories.rb
+++ b/app/jobs/regular/localize_categories.rb
@@ -17,7 +17,8 @@ module Jobs
       cat_id = args[:from_category_id] || Category.order(:id).first&.id
       last_id = nil
 
-      categories = Category.where("id >= ?", cat_id).order(:id).limit(BATCH_SIZE)
+      categories =
+        Category.where("id >= ? AND locale IS NOT NULL", cat_id).order(:id).limit(BATCH_SIZE)
       return if categories.empty?
 
       categories.each do |category|
@@ -26,9 +27,13 @@ module Jobs
           next
         end
 
-        CategoryLocalization.transaction do
-          locales.each do |locale|
-            next if CategoryLocalization.exists?(category_id: category.id, locale: locale)
+        locales.each do |locale|
+          localization = category.category_localizations.find_by(locale:)
+
+          if locale == category.locale && localization
+            localization.destroy
+          else
+            next if locale == category.locale
             begin
               DiscourseAi::Translation::CategoryLocalizer.localize(category, locale)
             rescue FinalDestination::SSRFDetector::LookupFailedError
diff --git a/app/jobs/scheduled/categories_locale_detection_backfill.rb b/app/jobs/scheduled/categories_locale_detection_backfill.rb
new file mode 100644
index 00000000..4ea30c00
--- /dev/null
+++ b/app/jobs/scheduled/categories_locale_detection_backfill.rb
@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+module Jobs
+  class CategoriesLocaleDetectionBackfill < ::Jobs::Scheduled
+    every 1.hour
+    sidekiq_options retry: false
+    cluster_concurrency 1
+
+    def execute(args)
+      return if !SiteSetting.discourse_ai_enabled
+      return if !SiteSetting.ai_translation_enabled
+      return if SiteSetting.ai_translation_backfill_rate == 0
+
+      categories = Category.where(locale: nil)
+
+      if SiteSetting.ai_translation_backfill_limit_to_public_content
+        categories = categories.where(read_restricted: false)
+      end
+
+      categories = categories.limit(SiteSetting.ai_translation_backfill_rate)
+      return if categories.empty?
+
+      categories.each do |category|
+        begin
+          DiscourseAi::Translation::CategoryLocaleDetector.detect_locale(category)
+        rescue FinalDestination::SSRFDetector::LookupFailedError
+        rescue => e
+          DiscourseAi::Translation::VerboseLogger.log(
+            "Failed to detect category #{category.id}'s locale: #{e.message}",
+          )
+        end
+      end
+
+      DiscourseAi::Translation::VerboseLogger.log("Detected #{categories.size} category locales")
+    end
+  end
+end
diff --git a/lib/translation/category_locale_detector.rb b/lib/translation/category_locale_detector.rb
new file mode 100644
index 00000000..544cf667
--- /dev/null
+++ b/lib/translation/category_locale_detector.rb
@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+module DiscourseAi
+  module Translation
+    class CategoryLocaleDetector
+      def self.detect_locale(category)
+        return if category.blank?
+
+        text = [category.name, category.description].compact.join("\n\n")
+        return if text.blank?
+
+        detected_locale = LanguageDetector.new(text).detect
+        locale = LocaleNormalizer.normalize_to_i18n(detected_locale)
+        category.update_column(:locale, locale)
+        locale
+      end
+    end
+  end
+end
diff --git a/lib/translation/category_localizer.rb b/lib/translation/category_localizer.rb
index d22a800f..1de93432 100644
--- a/lib/translation/category_localizer.rb
+++ b/lib/translation/category_localizer.rb
@@ -9,9 +9,12 @@ module DiscourseAi
         target_locale_sym = target_locale.to_s.sub("-", "_").to_sym
 
         translated_name = ShortTextTranslator.new(category.name, target_locale_sym).translate
-        # category descriptions are first paragraphs of posts
         translated_description =
-          PostRawTranslator.new(category.description, target_locale_sym).translate
+          if category.description.present?
+            PostRawTranslator.new(category.description, target_locale_sym).translate
+          else
+            ""
+          end
 
         localization =
           CategoryLocalization.find_or_initialize_by(
diff --git a/spec/jobs/regular/localize_categories_spec.rb b/spec/jobs/regular/localize_categories_spec.rb
index 5f778cc7..e0e55d18 100644
--- a/spec/jobs/regular/localize_categories_spec.rb
+++ b/spec/jobs/regular/localize_categories_spec.rb
@@ -53,7 +53,9 @@ describe Jobs::LocalizeCategories do
   end
 
   it "translates categories to the configured locales" do
+    Category.update_all(locale: "en")
     number_of_categories = Category.count
+
     DiscourseAi::Translation::CategoryLocalizer
       .expects(:localize)
       .with(is_a(Category), "pt")
@@ -69,20 +71,19 @@ describe Jobs::LocalizeCategories do
   it "skips categories that already have localizations" do
     localize_all_categories("pt", "zh_CN")
 
-    category1 =
-      Fabricate(:category, name: "First Category", description: "First category description")
-    Fabricate(:category_localization, category: category1, locale: "pt", name: "Primeira Categoria")
-
-    # It should only translate to Chinese, not Portuguese
-    DiscourseAi::Translation::CategoryLocalizer.expects(:localize).with(category1, "pt").never
-    DiscourseAi::Translation::CategoryLocalizer.expects(:localize).with(category1, "zh_CN").once
+    DiscourseAi::Translation::CategoryLocalizer.expects(:localize).with(is_a(Category), "pt").never
+    DiscourseAi::Translation::CategoryLocalizer
+      .expects(:localize)
+      .with(is_a(Category), "zh_CN")
+      .never
 
     job.execute({})
   end
 
   it "continues from a specified category ID" do
-    category1 = Fabricate(:category, name: "First", description: "First description")
-    category2 = Fabricate(:category, name: "Second", description: "Second description")
+    category1 = Fabricate(:category, name: "First", description: "First description", locale: "en")
+    category2 =
+      Fabricate(:category, name: "Second", description: "Second description", locale: "en")
 
     DiscourseAi::Translation::CategoryLocalizer
       .expects(:localize)
@@ -99,7 +100,7 @@ describe Jobs::LocalizeCategories do
   it "handles translation errors gracefully" do
     localize_all_categories("pt", "zh_CN")
 
-    category1 = Fabricate(:category, name: "First", description: "First description")
+    category1 = Fabricate(:category, name: "First", description: "First description", locale: "en")
     DiscourseAi::Translation::CategoryLocalizer
       .expects(:localize)
       .with(category1, "pt")
@@ -110,6 +111,8 @@ describe Jobs::LocalizeCategories do
   end
 
   it "enqueues the next batch when there are more categories" do
+    Category.update_all(locale: "en")
+
     Jobs.run_later!
     freeze_time
     Jobs::LocalizeCategories.const_set(:BATCH_SIZE, 1)
@@ -134,10 +137,8 @@ describe Jobs::LocalizeCategories do
   it "skips read-restricted categories when configured" do
     SiteSetting.ai_translation_backfill_limit_to_public_content = true
 
-    category1 = Fabricate(:category, name: "Public Category", read_restricted: false)
-    category2 = Fabricate(:category, name: "Private Category", read_restricted: true)
-
-    DiscourseAi::Translation::CategoryLocalizer.expects(:localize).at_least_once
+    category1 = Fabricate(:category, name: "Public Category", read_restricted: false, locale: "en")
+    category2 = Fabricate(:category, name: "Private Category", read_restricted: true, locale: "en")
 
     DiscourseAi::Translation::CategoryLocalizer
       .expects(:localize)
@@ -150,4 +151,40 @@ describe Jobs::LocalizeCategories do
 
     job.execute({})
   end
+
+  it "skips creating localizations in the same language as the category's locale" do
+    Category.update_all(locale: "pt")
+
+    DiscourseAi::Translation::CategoryLocalizer.expects(:localize).with(is_a(Category), "pt").never
+    DiscourseAi::Translation::CategoryLocalizer
+      .expects(:localize)
+      .with(is_a(Category), "zh_CN")
+      .times(Category.count)
+
+    job.execute({})
+  end
+
+  it "deletes existing localizations that match the category's locale" do
+    # update all categories to portuguese
+    Category.update_all(locale: "pt")
+
+    localize_all_categories("pt", "zh_CN")
+
+    expect { job.execute({}) }.to change { CategoryLocalization.exists?(locale: "pt") }.from(
+      true,
+    ).to(false)
+  end
+
+  it "doesn't process categories with nil locale" do
+    # Add a category with nil locale
+    nil_locale_category = Fabricate(:category, name: "No Locale", locale: nil)
+
+    # Make sure our query for categories with non-null locales excludes it
+    DiscourseAi::Translation::CategoryLocalizer
+      .expects(:localize)
+      .with(nil_locale_category, any_parameters)
+      .never
+
+    job.execute({})
+  end
 end
diff --git a/spec/jobs/scheduled/categories_locale_detection_backfill_spec.rb b/spec/jobs/scheduled/categories_locale_detection_backfill_spec.rb
new file mode 100644
index 00000000..5eff85fd
--- /dev/null
+++ b/spec/jobs/scheduled/categories_locale_detection_backfill_spec.rb
@@ -0,0 +1,103 @@
+# frozen_string_literal: true
+
+describe Jobs::CategoriesLocaleDetectionBackfill do
+  fab!(:category) { Fabricate(:category, locale: nil) }
+  subject(:job) { described_class.new }
+
+  before do
+    SiteSetting.discourse_ai_enabled = true
+    Fabricate(:fake_model).tap do |fake_llm|
+      SiteSetting.public_send("ai_translation_model=", "custom:#{fake_llm.id}")
+    end
+    SiteSetting.ai_translation_enabled = true
+    SiteSetting.ai_translation_backfill_rate = 100
+  end
+
+  it "does nothing when AI is disabled" do
+    SiteSetting.discourse_ai_enabled = false
+    DiscourseAi::Translation::CategoryLocaleDetector.expects(:detect_locale).never
+
+    job.execute({})
+  end
+
+  it "does nothing when content translation is disabled" do
+    SiteSetting.ai_translation_enabled = false
+    DiscourseAi::Translation::CategoryLocaleDetector.expects(:detect_locale).never
+
+    job.execute({})
+  end
+
+  it "does nothing when backfill rate is 0" do
+    SiteSetting.ai_translation_backfill_rate = 0
+    DiscourseAi::Translation::CategoryLocaleDetector.expects(:detect_locale).never
+
+    job.execute({})
+  end
+
+  it "does nothing when there are no categories to detect" do
+    Category.update_all(locale: "en")
+    DiscourseAi::Translation::CategoryLocaleDetector.expects(:detect_locale).never
+
+    job.execute({})
+  end
+
+  it "detects locale for categories with nil locale" do
+    DiscourseAi::Translation::CategoryLocaleDetector.expects(:detect_locale).with(is_a(Category)).times(Category.count)
+
+    job.execute({})
+  end
+
+  it "handles detection errors gracefully" do
+    DiscourseAi::Translation::CategoryLocaleDetector.expects(:detect_locale).with(is_a(Category)).at_least_once
+    DiscourseAi::Translation::CategoryLocaleDetector
+      .expects(:detect_locale)
+      .with(category)
+      .raises(StandardError.new("error"))
+      .once
+
+    expect { job.execute({}) }.not_to raise_error
+  end
+
+  it "logs a summary after running" do
+    DiscourseAi::Translation::CategoryLocaleDetector.stubs(:detect_locale)
+    DiscourseAi::Translation::VerboseLogger.expects(:log).with(includes("Detected #{Category.count} category locales"))
+
+    job.execute({})
+  end
+
+  describe "with public content limitation" do
+    fab!(:private_category) { Fabricate(:private_category, group: Group[:staff], locale: nil) }
+
+    before do
+      # catch-all for other categories
+      DiscourseAi::Translation::CategoryLocaleDetector.expects(:detect_locale).with(is_a(Category)).at_least_once
+
+      SiteSetting.ai_translation_backfill_limit_to_public_content = true
+    end
+
+    it "only processes public categories" do
+      DiscourseAi::Translation::CategoryLocaleDetector.expects(:detect_locale).with(category).once
+      DiscourseAi::Translation::CategoryLocaleDetector.expects(:detect_locale).with(private_category).never
+
+      job.execute({})
+    end
+
+    it "processes all categories when setting is disabled" do
+      SiteSetting.ai_translation_backfill_limit_to_public_content = false
+
+      DiscourseAi::Translation::CategoryLocaleDetector.expects(:detect_locale).with(category).once
+      DiscourseAi::Translation::CategoryLocaleDetector.expects(:detect_locale).with(private_category).once
+
+      job.execute({})
+    end
+  end
+
+  it "limits processing to the backfill rate" do
+    SiteSetting.ai_translation_backfill_rate = 1
+    Fabricate(:category, locale: nil)
+
+    DiscourseAi::Translation::CategoryLocaleDetector.expects(:detect_locale).once
+
+    job.execute({})
+  end
+end
diff --git a/spec/lib/translation/category_locale_detector_spec.rb b/spec/lib/translation/category_locale_detector_spec.rb
new file mode 100644
index 00000000..078b9ecf
--- /dev/null
+++ b/spec/lib/translation/category_locale_detector_spec.rb
@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+describe DiscourseAi::Translation::CategoryLocaleDetector do
+  describe ".detect_locale" do
+    fab!(:category) { Fabricate(:category, name: "Hello world", description: "Welcome to this category", locale: nil) }
+
+    def language_detector_stub(opts)
+      mock = instance_double(DiscourseAi::Translation::LanguageDetector)
+      allow(DiscourseAi::Translation::LanguageDetector).to receive(:new).with(
+        opts[:text],
+      ).and_return(mock)
+      allow(mock).to receive(:detect).and_return(opts[:locale])
+    end
+
+    it "returns nil if category is blank" do
+      expect(described_class.detect_locale(nil)).to eq(nil)
+    end
+
+    it "updates the category locale with the detected locale" do
+      text = "#{category.name}\n\n#{category.description}"
+      language_detector_stub({ text: text, locale: "zh_CN" })
+
+      expect { described_class.detect_locale(category) }.to change { category.reload.locale }.from(nil).to(
+        "zh_CN",
+      )
+    end
+
+    it "handles category with no description" do
+      no_description_category = Fabricate(:category, name: "Test Category", description: nil, locale: nil)
+      language_detector_stub({ text: no_description_category.name, locale: "fr" })
+
+      expect { described_class.detect_locale(no_description_category) }.to change { no_description_category.reload.locale }.from(nil).to("fr")
+    end
+
+    it "bypasses validations when updating locale" do
+      language_detector_stub({ text: "#{category.name}\n\n#{category.description}", locale: "zh_CN" })
+
+      described_class.detect_locale(category)
+      expect(category.reload.locale).to eq("zh_CN")
+    end
+  end
+end