、。
People who praise you to your face often say bad things behind your back.
Literal
Face-to-face praise-like [possessive-の] person [topic-は], tendency hidden [at-で] bad-speak [thing-もの] is.
とかく ('tends to, is inclined to, often') is a written/formal adverb flagging a habitual tendency. 悪口を言う ('to speak ill of, to badmouth') is the standard collocation — literally 'to say evil words.' ~ものだ at the end is the generalising frame 'it is the case that X, X is how things go' — for proverbs and aphorisms. 陰で ('behind the back,' literally 'in the shadow') is another cultural idiom for gossip context.