You should have asked Expats too :) in my opinion... my former colleagues in Bangkok and many people in the travel industry speak excellent English and even another language like German on a high level. But people that rarely have to use English and don't work with foreigners don't need English and therefore don't practise English daily. It's similar in Germany (my home). Students and people in the travel industry will speak decent English but the general public not so much.
Just would like to give some suggestion, when you do the interview especially the foreigners asking about age is seem to be impolite in terms of culture different and you can see their face expressions. Pls be aware. By the way is a good content though.