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.