How do they know the Thing can talk?
It's the same kind of mistake they make in Star Wars, Star Trek and a million other sci-fi shows, when Spock or an android says, "Our plan of action only has a 2.33276 percent chance of success, Captain." Malarkey.
Worse, the characters assume that after transforming into a human (or infecting a human), that person will be able to speak English. Sorry, but cells can't tell which language you speak. Or to put it another way, if you were genetically engineering a creature to somehow mimic another creature perfectly, it would be one step to get the appearance of the host creature, but much harder to make it absorb the mind or memories or knowledge of the host. Maybe it would access memories and knowledge if it infected a fully living human, but to change the victim's attitude would still require a pretty complex parasite.
If you rely on decades of watching or reading sci-fi stories about shape-shifting alien doppelgangers, the coolest kind are the ones who get into your memories and talk. But other than following sci-fi tropes, there's no good reason to assume that an organism would be able to do that.
... I realize that we see characters who are later revealed as converted alien or infected human, so we eventually know that they can speak English. The question is: on what basis do all the characters jump to this conclusion before they have evidence of it?