If it’s an existing language then it’s not secure.

If you make up your own language you’ll waste a thousand hours and someone might use the notes you used to learn it.

Just use damn encryption, it’s easy and fast. Also has additional benefits. If someone wants to force you to give them/translate your notes you can’t do much (they’ll know when your translation is inconsistent).

With encryption you can hide a volume inside another one. If you enter the weaker password you can put decoy files there, with just your run of the mill notes. While behind a stronger password you hide your actual diary.