I’m not sure where this topic should go or if there has been other topics like this already… so please feel free to redirect me.
Anyway, I was thinking that maybe the reason why some DC don’t speak when spoken to and instead they walk away or just stare is because they don’t understand the language we’re speaking… 
What do you guys think?
Well, your subconscious makes up your DCs, and it probably speaks the same language you do. It might just be that your SC could be lazy or something 
. Actually, its probally it has nothing to say…
I mean, my DCs normally don’t speak French…but, I don’t think thats what your asking.
Well, i think that maybe when you’re concious about certain part of your mind while being lucid, that specific part you enlighten is the dc portrayed.
In other words, most dc are subconcious part of your mind that you have not yet awaken, when you do that, they sort of become of your concious and they won’t respond, is like talking in the mirror with yourself conciously, you won’t get any response.
Maybe they are just rude.
People do that in real life, is it so unlikely that your mind picks that up and puts them in the dream world?
I suppose it really depends on the questions, but I personally have never come across a DC that didn’t talk to me if spoken to. I have had some speak other languages at me though, French, German, Klingon.
go with Mohegen’s and AventuradeSueno’s answers, that’s what I basicly ment, but A LOT better and easier to understand 
Eh, usually it works the other way for me - I won’t speak when spoken to. Mainly because the arrogant subconscious part of me feels that the DC’s question validates no answer at the moment.
And sometimes I’m just ignorantly shy. (Because what if they did that to me? That leaves me in quite the causal loop.)
Actually, I’ve never actually got those non-talking DCs o_o; Not that I can remember, anyway.
Maybe i misunderstood you, but for me it’s not always the case. In most cases (at least in LD-s) when i’m concious about a DC i encounter, and when i’m very keen to listen what (s)he might say (and i want to remember it, hoping that it might be something smart), then they usually are talking to me (sometimes nonsense, sometimes something sensible and even smart). But sometimes it indeed happens, that when i’m very keen to listen DC’s reply, (s)he keeps silence.
Perhaps we can make them speek through means of torture 
*Rights this down on random notepad