FEATURE: Allow setting locale from 'lang' param (#30952)

As we start to translate more pages, we'll need a way for other sites to
link back to our translated topics.

This commit gives us the ability to use the "lang" URL param to define what
language a site should be in.

Related: https://github.com/discourse/discourse-translator/pull/199

spec/requests/application_controller_spec.rb

@@ -1126,6 +1126,44 @@ RSpec.describe ApplicationController do
         end
       end
     end
+
+    context "with set_locale_from_param enabled" do
+      context "when param locale differs from default locale" do
+        before do
+          SiteSetting.allow_user_locale = true
+          SiteSetting.set_locale_from_param = true
+          SiteSetting.default_locale = "en"
+        end
+
+        context "with an anonymous user" do
+          it "uses the locale from the param" do
+            get "/latest?lang=es"
+            expect(response.status).to eq(200)
+            expect(locale_scripts(response.body)).to contain_exactly("/assets/locales/es.js")
+            expect(I18n.locale.to_s).to eq(SiteSettings::DefaultsProvider::DEFAULT_LOCALE) # doesn't leak after requests
+          end
+        end
+
+        context "when the preferred locale includes a region" do
+          it "returns the locale and region separated by an underscore" do
+            get "/latest?lang=zh-CN"
+            expect(response.status).to eq(200)
+            expect(locale_scripts(response.body)).to contain_exactly("/assets/locales/zh_CN.js")
+          end
+        end
+      end
+
+      context "when locale param is not set" do
+        it "uses the site default locale" do
+          SiteSetting.allow_user_locale = true
+          SiteSetting.default_locale = "en"
+
+          get "/latest"
+          expect(response.status).to eq(200)
+          expect(locale_scripts(response.body)).to contain_exactly("/assets/locales/en.js")
+        end
+      end
+    end
   end
 
   describe "vary header" do