It’s OK but it doesn’t flow all that well and native speakers like me would juggle the words around and say it this way...”I don’t know when I’ll be in Japan next, but I’ll tell you when I’ve decided.” But always be aware that opinions will vary on this.

No: "I don't know when I'll next be in Japan, but I'll tell you when I've decided" "When I'll next be" = you'll definitely be in Japan again, so you will have to decide when to go. Your first sentence suggests you might not go to Japan again.