Quote 1
[Marlow to the Intended]: "'The last word he pronounced was - your name.'" (3.85)
The fact that Marlow says this and the Intended believes him is partially due to the fact that names constitute a very important part of language. They are an indication of identity.
Quote 2
"Suppose he [Kurtz] began to shout? Though he could hardly stand, there was still plenty of vigour in his voice." (3.28)
Marlow recognizes that Kurtz’s voice, the only strong thing about him, can still be a vehicle of communication. Thus, it poses a danger to him.
Quote 3
"'You made notes in Russian?' I asked. He nodded. 'I thought they were written in cipher,' I said." (2.37)
Language does not function well in the interior. For example, Marlow mistakes the harlequin’s Russian notes for cipher simply because he cannot read them.