What are all the languages supported by Siri as of Apple iOS 7.1? Is there a changelog available? I was able to find this information from the official support documentation for Siri: http://support.apple.com/kb/ht4992


  Siri understands and can speak the following languages:
  
  
  United States (English, Spanish)
  United Kingdom (English)
  Australia (English)
  France (French)
  Germany (German)
  Japan (Japanese)
  Canada (Canadian French, English)
  China (Mandarin)
  Hong Kong (Cantonese)
  Italy (Italian)
  Korea (Korean)
  Mexico (Spanish)
  Spain (Spanish)
  Switzerland (French, German, Italian)
  Taiwan (Mandarin)
  


But I don't think it has been updated as of iOS 7.1 (the page shows Last Modified: Sep 18, 2013). So...


What are all the languages that Siri supports as of iOS 7.1?
Is there an changelog available showing languages added in each version right from the beginning?
There is a change log available, you can find it here.
This is what it says about siri:

Siri


Manually control when Siri listens by holding down the home button
while you speak and releasing it when you're done as an alternative
to letting Siri automatically notice when you stop talking
New, more natural sounding male and female voices for Mandarin
Chinese, UK English, Australian English, and Japanese


And the change log you mentioned isn't updated because there was no need for it, I verified on my iPhone running iOS 7.1 and there are no new languages in addition to the list you posted.

On the developer portal there is another change log (here) that shows all changes in different versions.
It is a lot more technical than the "consumer log".