+1 to all suggestions above.
Also, is that lack of speaking skills apparent only when discussing issues around the particular field - design in the case you describe? If it is, it may be because some people, while very good at what they are doing, can't really appreciate how brilliant they are, because to them it's just 'normal' and they don't think there is much to talk about. If that were the case here, then perhaps it would help if they could talk to someone who will help them see how gifted and good at 'their thing' they are? As an example, I am probably the least creative person you could imagine and just a thought that someone has all those ideas that they can translate into beautiful or practical or whatever other things makes me go 'wow' - I could not do that even if I tried.
I may be wrong in my perception, but I imagine that working as a designer requires a lot of 'deep in one's thoughts' creativity and can be an isolating process as such. I am not saying it is something bad (and I may well be wrong anyway), but simply if someone's work requires being on his/her own with their thoughts a lot, this may have an impact on how comfortable a person is discussing his/her ideas. It could be the case of 'I know how it all works and why, but I can't easily describe it, it just feels right'. Maybe it would help in such a case if a person practised by trying to explain to people - friends or family members - how the whole design process works, where the ideas come from, how they decide which one to go with etc. I, for one, would find that fascinating.