I guess this whole comment thread on the OP is a small additional evidence of how complex and subtle the grammar of a language is. It varies not only with the perspective and emphasis of the message, but also with the conventions of each dialect.
Perhaps "I'm currently in Thailand and I've been reading and writing mostly in English for about fourteen years." would be better? The unedited version which you found strange could be the case of erring on the side of oversimplifying the sentence grammar.
I upvoted your reply and all of the cousin replies too. Thanks for all of your responses.
Better, though to sound natural you'd want to drop the "in". (Also "be a case of" rather than "be the case of", and "some small additional evidence" or "a small additional piece of evidence" rather than "a small additional evidence").
(Hope none of this is too dispiriting; I'm being quite nitpicky)
Perhaps "I'm currently in Thailand and I've been reading and writing mostly in English for about fourteen years." would be better? The unedited version which you found strange could be the case of erring on the side of oversimplifying the sentence grammar.
I upvoted your reply and all of the cousin replies too. Thanks for all of your responses.