Encryption is defined as "the process of converting information or data into a code, especially to prevent unauthorized access."

What you describe is the same thing just not cryptographic.

> I wonder if we can make a chat app that doesn't use encryption but hides your messages inside random words.

This technique is called "chaffing and winnowing": https://en.wikipedia.org/wiki/Chaffing_and_winnowing

Format-Transforming Encryption (FTE) is what you're looking for--it ensures the resulting message isn't vulnerable to cryptanalysis techniques while enabling encoding of a message in arbitrary formats [0], including a blog post or text message. The authors of the source use it specifically to hide messages in semantically-correct network protocol packets, but it applies just as well to text.

[0] - https://dl.acm.org/doi/pdf/10.1145/2508859.2516657

> The solution should be saved locally on your device.

Installed as a signed app...?

There isn't a technology solution for this. The solution is to realize the value Switzerland lost with Proton moving out of Switzerland, and what Germany could lose if Wire had to make a similar decision regarding their home in Germany. There's considerable value to having real effective security. These nations stand to lose that value.

That's just an insecure form of encryption