Is is appropriate to put this on a shirt: 自分を信じて
自分を信じて is natural as something to print at least on a casual T-shirt. When it comes to jiujitsu-gi, however, I haven't seen a judo-gi with a message on it in the first place, so it might appear a bit strange to Japanese eyes. Still, if you really need to print something, 自分を信じて is not a strange message at all. It conveys the intended meaning ("Believe in yourself") well.