The other day...
My ma looked through my phone and told me I was texting people in a made Up language. I was and still am texting people in Indonesian. How rude. What to do??
Be glad you have a secret language that she can't see? Did she have your permission to look at your texts? If not then this was twice as rude.
If you really want to persuade her that it is real you could show some sentences on Duolingo or use a dictionary, but i wouldn't know why you would bother. If someones mind is closed...
(although in one sense she is correct, it is made up, like every other language on this earth!-)
Play Indonesian music out loud and sing to it while being seen, or set your phone/laptop wallpaper to a picture of somewhere in Indonesia with clear Indonesian writings, or cook her an Indonesian food, …
I completely agree with Leafhuntress' comment, btw.