Saw an interesting study today that had a test group of Chinese that had been living and studying the US for a short time. They measured their English fluency when conversing with a western face vs an asian face. When conversing with the asian face their fluency dropped though their comfort level rose. Inverse for the western face. The study extrapolates from the results that when we are in a more comfortable, more native environment (in this case, talking to a similar face), our fluency in the foreign language drops.

I guess to take it one step further, it would mean that when speaking Thai to your significant other in an environment that becomes "comfortable" or more like your native country, your Thai language skills would suffer. Or perhaps it means speaking Thai with a luuk-kruung or someone with a western face would be more difficult.

http://www.pnas.org/content/early/2013/06/19/1309472110.full.pdf+html

I don't think I'm very comfortable speaking Thai to other westerners.