Lines 12-23 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Practice until you feel
the language inside you, says the man.

We're back to that first conversation between father and son. The dad says his kid should have the goal of feeling the language inside of him, presumably because that's when he'll know he's getting good. Basically, it sounds like the father wants the kid to practice the foreign language until it doesn't feel foreign any more. Until it feels like a part of him. Anyone who's ever studied a foreign language knows that this will take a lot of practice.
We wonder if the hope is this: if the child takes the language inside of him, then the culture that uses that language might take root in the child, too, making him less foreign. That would be an example of the "assimilation" our speaker mentioned before.
The language takes a turn for the figurative here. Of course, a language can't literally be inside you, but if you think and speak in a language, it can feel like it's a part of your identity.

But what does he know about inside and outside,
my father who was spared nothing
in spite of the languages he used?

Looking back at that conversation, our speaker doubts what his father could have truly understood about inside and outside. He suggests that knowing other languages didn't end up saving his father any grief or suffering.
Okay, so now we know that the son and the man (who were in the first lines) are definitely our speaker and his father, from some moment in the past, perhaps when our speaker was a child.
Even though our speaker is looking back at a moment when he was a child, he definitely is bringing his adult perspective to the table.
We feel like this happens a lot: as a kid we hear our parents or other adults say things that we kind of take for granted. We assume, because they're adults, that they must know what they're talking about. But then when we get older we start to realize: wait a minute... there's no age we hit when we suddenly inherit vast stores of knowledge and certainty. They were faking it!
Our speaker is able to realize just how little his dad knew because now he's an adult, and has to come to terms with just how little he knows.

And me, confused about the flesh and soul,

Our speaker turns from his father's confusion about inside and outside, to his own uncertainty about the body and soul. It seems like everyone's a bit confused in this poem, or at least everyone we've met so far.
At first this seems like a bit of a jump in the conversation, but when we think about it, this could also be seen as a confusion of outside (flesh) and inside (soul).
We're glad our speaker can also spot his own confusion and lack of understanding, and not just that of others. He seems like a pretty humble guy.

who once asked into a telephone,Am I inside you?

In his confusion, our speaker asks, while on the phone, "Am I inside you?"
Is he talking to just the phone in his hand or to the person on the other end? The phrasing could go either way, but we're going to go ahead and guess there's someone listening.
Still, the line also seems to play on how the voice of the person on a phone always sounds like it's inside the phone itself. (And, especially for a poet, a voice might be someone's very identity.)
But here's our question: what exactly does this have to do with our speaker's confusion about the flesh and soul? We're getting there, don't worry.

You're always inside me, a woman answered,
at peace with the body's finitude,
at peace with the soul's disregard
of space and time.

The woman on the phone with our speaker says yes to our speaker's question, and he thinks it's because she's at peace with mortality and the nature of the soul – the "inside."
We're guessing this lady might be his wife, or perhaps the mother of his child. She's definitely a love interest of some sort.
So what does she mean by her answer? Our guess is that she might be saying something like: I love you and think about you, so you're always inside me, in my thoughts and memories and feelings. But that's just Shmoop's opinion. What do you think she's saying?
It's definitely a little weird or mysterious, the way our speaker connects her answer to this declaration that she's at peace with death and how the soul relates to space and time. But we can see a connection with the calm and the patience of her answer, and the idea of not being troubled by those things.
Plus this helps us understand that our speaker, in contrast, is probably not at peace with those things. Maybe that's why he's so anxious and uncertain, and maybe that's why he asked the question in the first place.