Would I have understood facial expressions if had been raised without human contact?

If a tree falls in the Mariana Trench, does it make a noise?

I think laughing and smiling are built into us.

Tarzan did.

I thought I was making a gal do that once but turns out the Mexican dinner we had was just doing a number on her

Some, yes.

Smiles, frowns, etc, are universal

But others, are social constructs

How does the Brown-headed Cowbird know it is a Brown-headed Cowbird?

This post on X shows imaging (MRI maybe?) of a baby in the womb making both a smile and cry face, supposedly in response to exposure to different foods:

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I don't know where she sourced this from, but if it's true, it clearly suggests the ability and desire to make facial expressions are innate since the baby could not have visually learned them from anyone.

As to whether a person can understand them on another person without training or exposure, I think the answer is yes, with a caveat...

I remember a time or two when I've been in a bad mood and been around others who were in a happy, carefree state and at that moment I was not vibing with them in the least. It might be a slight stretch to say I was in a temporary dissociative state in relation to these happy people, but in such moments I had no empathy with their feelings, thus saying I was in a dissociative state seems close enough for my point. It follows, then, that if someone remains in a dissociative state, they might not truly understand the expressions of others.

To be clear, I could recognize the happiness in others in these rare moments, but I was not in a state of truly understanding it, and may have even thought some of them to be foolish or had some other insult in mind with which I regarded them. I'm making a distinction between recognition and understanding. To truly understand, you have to have a level of empathy. As a parallel, consider a movie or book character who is a grouch of sorts most of the way through the story (like Scrooge in A Christmas Carol), but has an epiphany that allows him to finally understand the people around him. This sort of character transformation effectively changes the way people see and understand the world.

If you stretch your hypothetical question to its absolute limits, it would theoretically be possible for someone to not recognize or understand facial expressions on others. But it would require that person to also have a severely stunted development. They would have to have lived in a way that they did not experience emotions that induced expressions on their own face in a way they could reflect on and understand personally. And then they would have to be in a sort of permanent dissociative state with an utter lack of empathy that prevented them from ever resonating with the feelings of others.

I want to be as high as you are