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

config/locales/server.en.yml

@@ -1737,6 +1737,7 @@ en:
     allow_user_locale: "Allow users to choose their own language interface preference"
     set_locale_from_accept_language_header: "set interface language for anonymous users from their web browser's language headers"
     set_locale_from_cookie: "Allows setting an anonymous user's locale via the 'locale' browser cookie"
+    set_locale_from_param: "Allows setting an anonymous user's locale via the 'lang' URL param, e.g. ?lang=es"
     support_mixed_text_direction: "Support mixed left-to-right and right-to-left text directions."
     min_post_length: "Minimum allowed post length in characters (excluding personal messages)"
     min_first_post_length: "Minimum allowed first post (topic body) length (excluding personal messages)"

config/site_settings.yml

@@ -154,6 +154,10 @@ basic:
     default: false
     client: true
     validator: "AllowUserLocaleEnabledValidator"
+  set_locale_from_param:
+    default: false
+    client: true
+    validator: "AllowUserLocaleEnabledValidator"
   support_mixed_text_direction:
     client: true
     default: false

lib/discourse.rb

@@ -1215,13 +1215,11 @@ module Discourse
   end
 
   def self.anonymous_locale(request)
-    locale =
-      HttpLanguageParser.parse(request.cookies["locale"]) if SiteSetting.set_locale_from_cookie
+    locale = request.params["lang"] if SiteSetting.set_locale_from_param
+    locale ||= request.cookies["locale"] if SiteSetting.set_locale_from_cookie
     locale ||=
-      HttpLanguageParser.parse(
-        request.env["HTTP_ACCEPT_LANGUAGE"],
-      ) if SiteSetting.set_locale_from_accept_language_header
-    locale
+      request.env["HTTP_ACCEPT_LANGUAGE"] if SiteSetting.set_locale_from_accept_language_header
+    HttpLanguageParser.parse(locale)
   end
 
   # For test environment only

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