Even when I was in the last year of high school when my English was not very good, I never imagined of thinking in Thai before writing :-)-paragraph essay (that was the most we could do in those days). I thought English, like any other subject, exists in its own right. Whenever I did something in English, I did not even compare it in Thai. The same could be said when I studied biology, chemistry, physics etc. I concentrated on that particular subject in that particular environment.
When I thought of a "chair", I did not think as "เก้าอี้" but formed the image in my mind and put the symbol "CHAIR" as description.
When I thought of "walking", I did not think in Thai first as "เดิน" but I imagined the steps it took me to move forward.
If I could do this over 40 years ago when there was practically no advanced technology to help in studying English, I could not imagine that a Thai student cannot do as I did. You only need to study hard, read a lot of graded readers/grammar books/do exercises, and listen a lot (in those days I listened to short-wave radio BBC and VOA which was very hard on my ears as there were plenty of static noises). The only thing missing in those days was "writing". I did not write enough back then.