What standard of English are Air Traffic Controllers held to?
You might want to have a look into ICAO Language Proficiency Rating Scale. The article suggests that


  In 2003, ICAO set a deadline of March 2008 for English language
  proficiency at Level 4 and above for all pilots flying international
  routes and air traffic controllers serving international airports and
  routes.


Original text is not in bold.

In the same link, you can also find the description of each rating scale, as described in the Manual on the Implementation of ICAO Language Proficiency Requirements Doc 9835 AN/453, quoted here for convenience:



As of translators on call, I've never heard of such a practice and if I judge from the Level 4 being the minimum, that wouldn't be necessary.