There are quite a few thread titles, posts, and job ads. on Forumosa which use the term “ESL”.
“ESL” is generally understood to mean “English as a Second Language”.
In the same way, learning English in Taiwan is generally Foreign Language learning; EFL, whereas learning it in the USA, UK etc. is Second Language learning; ESL.
The term “ESL” is sometimes used by schools in Taiwan to refer to subject teaching in English; for example teaching geography in English so that the students learn both English and geography. This usage is an understandable marketing strategy, but I still feel that “EFL” is the appropriate term. And for buxiban classes, etc, where all that is being taught is English then there is no excuse to use anything other than “EFL”.
I don’t like to play the part of the language police (which is why I have started a separate thread instead of sniping at posters in other threads). I feel that the term “ESL” is being misused in the majority of cases, however, and that as language teaching professionals we should pick our terms carefully.