When do yoy introduce a new language?

First of all thanks for every advice in the prev topic- I started a new because I’m really interested. The question is: how long do you wait before you start to introduce a new language? I mean with the 3rd, 4th… Is that enough if they just get used to listen and get familiar with the sound of it or they need more, deeper “knowledge” before the next one. I’m almost ready to start his 3rd leng, that will be spanish and than (I don’t know when) I would like to go with Chinese.
By the way, What do you think: Chinese or Japanese?
Thanks