他只是开玩笑而已,不要把他的话放在心里 英文怎么说?

he's kidding. Don't bother.

最口语化和最简捷的译法。

He "was" just kidding. Do not take it serious.

楼上的注意语法， 他只是开玩笑而已,已经发生了，用过去时。 Do not take it serious. 别当真。
美国人也说，

He was just teasing you. 他是逗你玩。因为老美很爱开玩笑。

He was just kidding/joking, don't take it seriously.
或者说don't take it to the heart也可以。前者较常见。

He was just joking, but not to his words on the mind

just kidding

两个单词足以

He was just kidding. Do not take his words seriously.