Do you speak more than one language? Fluently? I speak 3 languages fluently, and know a few others to certain degrees. How long do you think it takes to learn a language fluently enough that you can use it for most stuff you might want to write? And even if you choose a weird or obscure language that will make it harder for you to learn, and if someone wants to read it all they need to do is pay any person who also speaks it.

If on the other hand you want to invent your own language and writing system there won’t be any other speakers to consult, however inventing your own language will take you thousands of hours, and if you only speak english languages your created will likely just be code words for everything because you don’t know about things like declinations or gender. As for the writing it’s easier to create a hard to crack writing system, however at the end of the day your system will either be semantic or phonetic, i.e. a symbol will represent either a meaning or a sound, if each symbol is a meaning you’ll need thousands of them which means thousands of hours to create and learn just a basic setup, if on the other hand you use phonetic symbols it’s almost a guarantee your language will be cracked simply because of repeating patterns. So the best case scenario is that you spend the next 5 years developing and learning a new language, and it would take a couple of months, best case scenario some years for someone to break it.

Let’s compare that with cryptography, you only need to remember a master key, if that master key is complex enough, e.g. 12 random dictionary words (which you can use several tools to generate), it would take all of the computer power in the world a couple of billion years to crack. Quantum computers might speed that up, but there are algorithms that work around that. How long does it take you to memorize 12 words? A couple of hours maybe, and you have something which is, in any meaningful use of the word, unbreakable. That seems like a much, much better deal.

And this comes back to an idea in computer science which is “don’t reinvent the wheel”, whatever you think is your great idea to encrypt your messages is likely already been thought of and discarded by the people who understand the most advanced cryptography algorithms today, and if you understood them you wouldn’t need to think on writing foreign languages.