It's appropriate that I was struck by the language in The Mission Song - after all, the central character is a translator. Perhaps that's why John le Carre used lyric cadences which washed over me, pulling me into the novel long before I became engrossed in the plot. Bruno Salvador (Salvo), is the illegitimate son of an Irish priest and an African woman. Raised in a series of missions and schools, his natural ear for languages has made him the top translator in London, specializing in African dialects. Married to a socially prominent rising tabloid editor (bringing to mind a blonde Rebekah Brooks), he translates for whomever needs him, including MI6.
That's the "important client" which calls Salvo away from the black tie event celebrating his wife's promotion. They need Salvo to translate (and not translate) during a delicate negotiation among several African leaders. It's the "not translate" which piques Salvo's interest. He's a translator, not just translating but interpreting what he hears, and he just can't turn that off. He can't pretend to not understand all he understands, and when he learns that the plan MI6 has formulated will harm rather than help Africa, he can't let the plan stand. Especially since he's fallen in love with a nurse from his mother's native region. Salvo tries to undo some of the damage created by the meeting where he translated (and didn't translate), but he's naive and not as well connected as he thought. It ends badly for him (although with some consolations), but because the language le Carre used is so beautiful, it's easy as a reader to minimize that fact.