If I won't go to work tomorrow because I've applied for leave, is it idiomatic to say "I'll be on leave tomorrow"? I googled this expression but found a lot of hits are from international sites
Yes, it's fine in BrE. You could also say:
I'm on annual leave tomorrow.
I've got annual leave tomorrow.
I'm away tomorrow.
I've got the day off tomorrow.
I'm on holiday tomorrow.