I honestly think that most introverts have their own "thought language" that works well for them, but has to be translated into "human language" when they speak. So it never hurts to dedicate lots of time to thinking conversationally, just as practice. You must already be doing this, though, or you never would have been able to learn German.
Wow, now that you mention "conversational thinking", it's really hard for me to try inventing them. It's like my mind is automatically thinking only about the given topic rather than how it should be discussed.
Actually, with German, I don't invent conversations, but when I'm reading or when someone says something, I often try to translate it.
About the "thought language" idea, I think that makes a lot of sense and indeed, I often find myself grasping for words and even whole sentences and ideas that might accurately portray my thoughts. Like you mentioned with "translation" into "human language", I have to do exactly that, which requires time, something you don't have much of in an actual talk. It's the same with this paragraph I just wrote; it took me way too much time to formulate a response even though I knew exactly what I wanted to express the moment I finished reading your post.
This. I actually started doing this pretending when I was little, but later I did it just out of a need to have some form of social stimulation, whether real or imaginary. In the long run, while I thought I was a loser for doing it, it really, really helped me be better at talking to people. It also helped me get over my stuttering problem. Do it when you're walking somewhere or doing something that doesn't require the whole of your brain.
Fuck, I hate stuttering. I've mostly gotten over that but mainly because I don't talk as much as before.
I used to be better at imagining conversations, but I think that maybe as I got older, my mind learned to cut out more and more words and phrases that aren't needed for comprehension, and cast all that into my subconscious. So I find it hard to think about people talking to each other unless it (the conversation) is being presented before me. And then when this happens, I go into "listening mode", trying to understand the core idea of what is being said, which explained differently means that my mind is going to translate it into my "thought-language." All of this takes time, so for example if I'm arguing with someone, I spend so much energy playing audience to his argument that I think what my counter would have been only after reflection in solitude, at which point it's usually too late. So it's almost as if, when in a conversation, I reduce my self to the status of "observer". Now the reality isn't quite that bad, but this is just my estimation of what happens.
EDIT: Did I just write another meandering, overanalyzed block of text? What the hell.