Perhaps you should familiarize yourself with the Private Language Argument by Wittgenstein.
Most people, I think, conceptualize of language like this:
You look at an apple, and have an impression of it in your mind, through your senses, etc and so forth. I look at the same apple, and have my own impressions. Now ... for all I know, when you look at the apple, you see what I would call a blue frog. Likewise, for all you know, when I look at the apple, I see what you would visualize as a green star. Yet ... we have the word "apple" that maps both of what we understand the same object to be, and thus, words are useful to us in order to be able to communicate. Those words map our ideas and concepts in our minds to each other and the focus.
So our concepts are like a private language, which we then translate into a common language. In other words, your blue frog is your private language ... as is my green star. However the public and common language we share calls it an "apple".
Wittgenstein's idea is that such an idea conceding a private language is flawed. There is no such thing as a private language in that context. A language understood by only one person, concerning their concepts, has no meaning to anyone else and cannot be translated. Literally. If it can be translated, then it's not a private language.
For example ... translating green star and blue frog into "apple" is still translating concepts we already understand: green, stars, the color blue, frogs, etc. It's still not a private language.
In order for it to be a private language, it is one that only *you* can make sense of, and cannot be translated so that I can understand it. It may even be non-static and changing constantly to where you don't remember it (if I understand the argument correctly). So it makes no sense to talk of private languages, or private experiences, etc ... talking about them is using language that is common and public.
At first glance ... one may ask, "So what ?" lol ... but in essence, this suggests that language is a social phenomenon. It's brought about by necessity. Whatever is going on in your head .. if there is a private language, it would be incomprehensible to me or anyone else other than yourself. Thus, communication between two people is not private, it is limited, a social construct, and discusses commonalities.
Thus, humans having a "secret language" is somewhat of an oxymoron. I think it is more correct, in this context, to suggest the humans have a secret experience.
I interpret the Private Language Argument to suggest that all personal experience is foundationally ineffable as it pertains to society.
