The Mission Song by John le Carré

Bruno Salvador has worked on clandestine missions before.
A highly skilled interpreter, he is no stranger to the Official Secrets Act. But this is the first time he has been asked to change his identity - and, worse still, his clothes - in service of his country.
Whisked to a remote island to interpret a top-secret conference between no-name financiers and the Congolese warlords, Salvo's excitement is only heightened by memoires of the night before he left London, and his life-changing encounter with a beautiful nurse named Hannah.
Exit suddenly the unassuming, happily married man Salvo believed himself to be. Enter in his place the pseudonymous Brian Sinclair: spy, lover - and perhaps even, hero