External becomes internal; explicit becomes implicit. We internalize what is, at first, external.
That's why, if we wish to develop the skill to think in Tibetan, first we must speak in Tibetan! And a great way to practice thinking to yourself in Tibetan is to start talking to yourself in Tibetan.
So the next time you're on a walk; doing household chores; making your grocery list; or simply thinking about your day, practice by doing it aloud, in Tibetan!