From the publisher:
When Earthman Gary Towler is off work, he is a pariah. For his task as chief interpretor for the corrupt and tyrannical nuls makes other humans avoid him as a traitor. Nor is he trusted by the three-armed mammoth rulers themselves, especially when they learned that an envoy was on the way from their distant planetary headquarters to investigate charges of corruption on Earth. For the leaders realized that Gary knew too much. When the humans leading the underground rebellion demanded Gary's aid or his life, he was caught between two untrustful forces. And his only way out was to make himself into a one-man third force against two worlds' plotters.

Published in the United States in 1961 as The Interpreter