While I'm sure it varies by territory, and that airports around more cosmopolitan areas have controllers better versed in English, I was wondering how well controllers have to know English. I imagine that there are places where all of the English they know is aviation-related. I can imagine that there are situations in which lack of knowledge of the language outside of memorized aviation vocabulary could be harmful, and situations in which a translator might be needed. How well versed in English are ATCs?
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.
Air traffic controllers need an ICAO English proficiency level 4 or above.
You can see what the different levels require here:
The English proficiency is tested when the ATC certificate is first issued, and then repeatedly after a number of years, depending on the level (the lower level you get, the more often you need to be tested).