Consider the possibility that the brain has not necessarily "concluded" what is being put into words, internally. That the monologue is the conclusion, which we are processing.

So we have two points of view, perspectives, or explanations. Yours is that there is some internal-mind code that is NOT language, which we are able to convert instantly to language to communicate with others. Most people then also have an internal monologue for unknown or orthogonal reasons, while some don't have it, without making any significant difference, because it is something like a side effect.

Mine is that the internal monologue is the thought itself. There is no other, hidden, code that can express the same complexity as the language that we use both to communicate with others and our own mind. That we don't need this internal monologue for a lot of things (consider driving a car), but it's a crucial mechanism for planning long term tasks, solving new problems, and what generally separates us from other intelligent life forms.

Not the poster you replied to. But maybe I should quickly define what I mean by inner monologue: thoughts in word form, utilizing language. I defined it this way because this is what most people seem to mean when they talk about an inner monologue.

In a sense, I do have an inner monologue (or dialogue, or multilogue), it's just not done in words unless I need to crystalize my thoughts into something I can communicate with others. My internal method of thinking is a language; it's just a non-verbal language that's only "spoken" by one person on Earth: me.

I am quite capable of planning long term tasks, solving novel problems, performing abstract thinking, etc., without using words at all. It does, however, mean I need to maintain a huge amount of state in my head. Serializing that state to words is very cumbersome for me, but extremely useful to help reduce the amount of state I need to keep in mind, so it's a tradeoff between spending the annoyingly long amount of time needed to linearize and serialize my thoughts to the outside world, and having more resulting mental bandwidth to deal with the more critical parts of a problem at the moment.

Story time... After I lost my language after the stroke, my mind seemed to have been shattered, like an ancient empire falling, breaking up into numerous squabbling kingdoms. I became aware of the different functions of my brain in a new way. In terms of language, to this day when I communicate with others, it feels like I think in my internal non-verbal language, then sort of toss it over the wall to another part of my brain which converts that to words.

When I was first re-learning how to communicate with others, I managed to "talk" that other part of my brain into round-tripping the translations. So I'd think a thought, the translator part would convert it to words, then the sister part would convert the words back to my own nascent internal language. My own internal language developed alongside my ability to communicate with others via this feedback loop.

I'd never heard of generative adversarial networks at the time, but that's basically what I was doing. Before managing to convince that part of my brain to create the feedback loop, I had barely managed to regain any words at all. Afterward, my vocabulary began recovering rapidly even as it became more feasible to express complex concepts in my new internal way of thinking.

One thing this GAN-style behavior made clear to me, was that language was a highly useful error-correction method. Each time the round trip garbled meaning in various ways, it clarified the sloppiness in my new thought style, shining a light on it that made it easier to see just how much further I needed to go to regain my ability to read, write, and speak.

Maybe if I'd had help regaining my language, I wouldn't have had to develop a unique internal "language" to be able to express thoughts in, and I'd natively think in words again. Hard to say. All I know is I was desperate to be able to think again, and I wasn't willing to wait until I regained my words, especially given how slow the process was initially.

So, only going from my own personal experiences, I wonder if internal language partially "evolved" as a means of thought error-correction, to review what one was about to say or do before saying or doing it. It's also obviously useful for working with others, and our ability to transfer knowledge even across centuries is remarkably useful, but I also wonder if inner monologue has been maintained in so much of the population even for internal thoughts because that error-correction allows easier self-reflection of our thought process?

Even I sometimes will say aloud the details of the problem I'm currently trying to solve as a means to double-check that I'm not missing anything obvious. I don't do it often, but it can help when I'm stuck. If the translator part of my brain can't figure out a good way to translate my thoughts into words, it's an excellent sign I'm overthinking the problem and need to refocus on less abstruse details.