I have been speaking a fair amount of German lately for one reason and another, both socially and professionally. I find casual conversation much easier, especially when well lubricated; my Bierdeutsch is super-fluent!
Delivering a professional presentation is completely different. It strikes me that speaking in one's non-primary language is like running in debug mode, at least in my experience.
First of all, I am conscious of various different threads, all running at the same time but at different speeds: what do I want to say, how am I going to phrase it, what is the word I want, make sure it isn't a "false friend", make sure the case of the adjective agrees with the noun that supports it, don't forget the verb at the end, … None of these are fully synced up, either (except at the height of Bierdeutsch), so there is also a monitor thread watching all of these other threads. Speaking on a serious subject for any length of time in a language you are not fully comfortable in is exhausting.
Interestingly, it seems that there is some reality behind the metaphor of debug mode. Certainly it seems that reactions in a non-primary language are more considered and less subject to empathy, according to a study in PLOS ONE: Your Morals Depend on Language.
This is a really interesting finding, if you think about it for a moment: our thoughts are dependent on our ability to express them.
At its extreme, of course, this turns into 1984's Newspeak. According to Orwell,
"the purpose of Newspeak was not only to provide a medium of expression for the world-view and mental habits proper to the devotees of IngSoc, but to make all other modes of thought impossible. Its vocabulary was so constructed as to give exact and often very subtle expression to every meaning that a Party member could properly wish to express, while excluding all other meaning and also the possibility of arriving at them by indirect methods."
Could this mean that it is also possible for us to train ourselves to be better people by expanding our vocabulary and our facility with it?
People have been known to worry about the impact of the internet in general, and social media in particular, on "culture", for want of a better word - but one overriding aspect of the internet is that to participate, you need at least a minimum level of comfort with language. To emerge and to excel, you need mastery.
In other words, Twitter and Facebook will save us - by forcing people to think.