I was a language instructor as a SSgt in the USAF at the defense language institute Korean school in Monterey, CA. One of my students, an Army Special Forces SFC with Thai language experience (I have none), was participating in a creative role play involving numbers and vocabulary commonly used with numbers.
He put on his army trench coat (class b uniform day) stood in front of the class, and proceeded to act like a guy who sells watches on the street. His sales pitch (for a tv commercial, I guess) went fairly well until he started to give his 555-nnnn phone number in Thai rather than Korean.
The other students an I who had been concentrating keenly to figure out just what the hell the student SFC had been saying were totally taken by surprise by what we heard and a good laugh was had by all. The SFC was clearly working hard to communicate his free-form message (compared to many other lessons which concentrate on formatted language transactions such as greetings, weather reports, etc.), and it was quite a scene when he slipped into another language.
Good times.