Use regional fallbacks from i18n (#607)

bridgetown-core/lib/bridgetown-core/concerns/site/localizable.rb

@@ -10,9 +10,7 @@ def locale
         I18n.load_path += Dir["#{in_source_dir("_locales")}/**/*.{json,rb,yml}"]
         I18n.available_locales = config[:available_locales]
         I18n.default_locale = locale
-        I18n.fallbacks = (config[:available_locales] + [:en]).uniq.to_h do |available_locale|
-          [available_locale, [available_locale, locale, :en].uniq]
-        end
+        I18n.fallbacks = [locale, :en].uniq
         locale
       end
     end

bridgetown-core/test/test_locales.rb

@@ -165,4 +165,40 @@ def reset_i18n_config
       end
     end
   end
+
+  context "fallback chain" do
+    setup do
+      reset_i18n_config
+      @site = resources_site
+      @site.process
+    end
+
+    should "include English for base language" do
+      assert_equal %i[de en], I18n.fallbacks[:de]
+      assert_equal %i[fr en], I18n.fallbacks[:fr]
+    end
+
+    should "include English and base language for regional locale" do
+      assert_equal %i[de-NL de en], I18n.fallbacks[:"de-NL"]
+      assert_equal %i[fr-CA fr en], I18n.fallbacks[:"fr-CA"]
+    end
+  end
+
+  context "fallback chain with different default locale" do
+    setup do
+      reset_i18n_config
+      @site = resources_site("default_locale" => :es, "available_locales" => %w[en es])
+      @site.process
+    end
+
+    should "include both the default language and English in the fallback chain" do
+      assert_equal %i[de es en], I18n.fallbacks[:de]
+      assert_equal %i[es en], I18n.fallbacks[:es]
+    end
+
+    should "include base language, default, and English for regional language" do
+      assert_equal %i[de-NL de es en], I18n.fallbacks[:"de-NL"]
+      assert_equal %i[es-MX es en], I18n.fallbacks[:"es-MX"]
+    end
+  end
 end