When did the Unsullied learn to speak the common tongue?

Question

When Daenerys first acquires the Unsullied in Game of Thrones, none of them knew how to speak the common tongue. This is primarily exampled by Grey Worm having to take many lessons from Missandei (ex. S04E04), as well as Daenerys always speaking Valyrian when she address them. At no point do we hear/see the Unsullied speaking the common tongue, aside from Grey Worm.

In S08E05, when Tyrion goes to speak with his captive brother, Jamie, the [implied to be] low ranking Unsullied soldiers can speak/understand the common tongue fairly well.

So, when/how did the Unsullied learn the common tongue?

Answer 1

There's no reason to think the entire Unsullied army speaks common:

• Tyrion, who has spent a lot of time around Unsullied now, assumed that these particular guardsmen wouldn't speak Common, and tried to speak to them in Valyrian, even though his Valyrian is appallingly bad.
• When they replied in Common, he looked surprised.
• We've never seen any other Unsullied speaking Common.
• These weren't just any Unsullied, they had been specifically assigned to guard a high-ranking prisoner who only speaks Common.

So, either the entire army had been taking secret intensive language lessons and Tyrion didn't notice, or, more likely, some Unsullied speak or learned Common, and those were the ones chosen to guard high value prisoners who only speak Common.

---

How do any Unsullied know it?

It's literally called "the common tongue", so it's not exactly uncommon. There are loads of ways:

• They've had years to learn it, under a queen who is always talking about some day going to and ruling a place where everyone speaks it. We saw in Season 5 that Unsullied are individuals, not robots, and they're starting to explore their own personal interests a little. If some are spending their salaries on a kiss and a cuddle in a brothel, others on gambling in a bar, it'd be weird if there were none who took an interest in the wider world and learnt a very common language that was likely to be useful.
• Some might have known it already. If it's "the common tongue", some would speak it from before they were taken as slaves. It'd also be good business for the masters of Astapor to teach various common languages to various Unsullied in case a merchant or whatever who speaks only that language wants some bodyguards his family can give orders to without a translator, and it'd make sense for "the common tongue" to be one of these common tongues.

They're not an army of robots or clones built in a factory, it'd be weird if none of them spoke what is apparently the most common language in the world.

Answer 2

The quotes are from a comment you made, which address the core of why you're expecting something different from what you're getting.

I have friends who have lived in English speaking countries for years, and they work in English speaking environments every day, and they also began learning English at 7th grade, and their English isn't as half as good as the Unsullied are with the common tongue.

Unless the linguistic difficulty is significant to the plot, there's no reason to have the scene be slowed down by having the guard needing Tyrion to repeat himself a few times before he understands it.

Secondly, these guards were specifically guarding a prisoner who only speaks the common tongue. If the common language isn't that well known amongst all troops, it stands to reason that they assign guards that are relatively proficient in the language (compared to the rest of the Unsullied).

Never said it was surprising, just that I've not seen a lick of evidence in the entire series of any Unsullied learning/speaking common tongue other than GW, so the Tyrion scene seemed quite out of place.

Other than Tywin's death scene, we also did not see characters taking a shit, but that doesn't mean it didn't happen off-camera. The same applies to sleeping, or travelling, or washing yourself, or ... Game of Thrones, just like any other show or movie, skips over the boring parts and shows you the interesting bits.

Seeing Grey Worm's language lessons acts as the indication that the Unsullied are learning the common tongue. There's no point in showing the same thing over and over for all the Unsullied. The point has been made, there's no point in showing additional scenes that add nothing further to the story.

If you had gotten what you're asking for, there would be a boatload of complaints about the screentime they wasted on boring and irrelevant things.