FEATURE: Add a onebox_locale site setting. (#30655)

Following on from f369db5ae9a29a23299dff5f14768167230b0b79, this change adds the ability to choose a custom locale to send to onebox providers.

If this setting is left blank, it will fall back to using default_locale.

lib/oneboxer.rb

@@ -152,7 +152,7 @@ module Oneboxer
   end
 
   def self.redis_cached_response_body_key(uri)
-    "CACHED_RESPONSE_#{SiteSetting.default_locale}_#{uri}"
+    "CACHED_RESPONSE_#{onebox_locale}_#{uri}"
   end
 
   # Parse URLs out of HTML, returning the document when finished.
@@ -677,6 +677,9 @@ module Oneboxer
       force_custom_user_agent_hosts: force_custom_user_agent_hosts,
       preserve_fragment_url_hosts: preserve_fragment_url_hosts,
       timeout: 5,
+      headers: {
+        "Accept-Language" => accept_language,
+      },
     }
 
     uri = URI(url)
@@ -705,4 +708,16 @@ module Oneboxer
 
     fd_options
   end
+
+  def self.onebox_locale
+    SiteSetting.onebox_locale.presence || SiteSetting.default_locale
+  end
+
+  def self.accept_language
+    if onebox_locale == "en"
+      "en;q=0.9, *;q=0.5"
+    else
+      "#{onebox_locale.gsub(/_/, "-")};q=0.9, en;q=0.8, *;q=0.5"
+    end
+  end
 end