The first is more indirect. We don't know who says that he is good at translating. This form is often used for obscuring the origins of actions ('The door was left open' vs. 'he left the door open'). It's known as the passive. The second one is the active - we know who says this, and so is more often used for claims we want to be sure of the origins of, like sources in academic writing. If you're finding this stuff difficult, maybe book a grammar lesson and we'll work through it together.