Add Lita.default_locale=.

lib/lita/common.rb

@@ -12,14 +12,29 @@ def load_locales(paths)
       I18n.reload!
     end
 
-    # Sets I18n.locale, normalizing the provided locale name.
+    # Sets +I18n.locale+, normalizing the provided locale name.
+    #
+    # Note that setting this only affects the current thread. Since handler
+    # methods are dispatched in new threads, changing the locale globally will
+    # require calling this method at the start of every handler method.
+    # Alternatively, use {Lita#default_locale=} which will affect all threads.
     # @param new_locale [Symbol, String] The code of the locale to use.
     # @return [void]
     # @since 3.0.0
     def locale=(new_locale)
       I18n.locale = new_locale.to_s.tr("_", "-")
     end
 
+    # Sets +I18n.default_locale+, normalizing the provided locale name.
+    #
+    # This is preferred over {Lita#locale=} as it affects all threads.
+    # @param new_locale [Symbol, String] The code of the locale to use.
+    # @return [void]
+    # @since 4.8.0
+    def default_locale=(new_locale)
+      I18n.default_locale = new_locale.to_s.tr("_", "-")
+    end
+
     # The absolute path to Lita's templates directory.
     # @return [String] The path.
     # @since 3.0.0

spec/lita_spec.rb

@@ -58,6 +58,13 @@
     end
   end
 
+  describe ".default_locale=" do
+    it "sets I18n.default_locale to the normalized locale" do
+      expect(I18n).to receive(:default_locale=).with("zh-TW")
+      described_class.default_locale = "zh_TW"
+    end
+  end
+
   describe ".redis" do
     let(:redis_namespace) { instance_double("Redis") }