"开朗" 英文可以说是 "cheerful" 或 "optimistic"。
"Cheerful" 意思是乐观愉快的，而 "optimistic" 意思是对未来充满信心的。
例句：
He is always so cheerful, it's hard not to feel happy around him. (他总是那么开朗，身边有他很难不高兴。)
She is an optimistic person and always looks on the bright side of things. (她是个乐观的人，总是看到事情的积极面。)