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

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