I have these monologues too, and they are far better written than anything that comes from my hands. So I am pretty sure I am composing in "mentalese" augmented with just enough English to fool me into thinking I am making good prose.

Speaking out loud while thinking is a good exercise, and more difficult than just thinking. But when I do it ends up in a conversational style, as if I was giving an talk to tech colleagues.

Also when I am in Germany, I find it very easy to "think in German" even though I can't speak properly.