OK I have read a variety of opinions on the topic of 'Why can't Thais speak English?' Some are correct but some are outright shallow.
I am a Thai, born and bred, and I have had some Eng-teaching experience. I used to be an IP lawyer, then quit to become a law professor; I am now teaching in Phisanulok but currently on a sabbatical leave. I developed a course called "English for lawyers" to teach law students practical English for professional purposes (for a legal profession).
As for my language background, I speak rather good English, I have an intermediate command of French, basic Chinese, and have taught myself ancient Greek. Here are my observations and criticisms:
1. I think most Thai have a working knowledge of English. The ability to use English among Thai population is known to be better than in Japan or China. Thailand is a very popular tourist destination, it is not a secret that people here use English well in their professional lives.
2. I never had any problem creating motivation for my students. As a Thai, I know that mastery of a foreign language requires that 95% of the work be done outside of the classroom. So, my foremost goal has always been to create motivation and love of learning English.
I have a realistic expectation and always tell my student that they will not walk away from the course being able to use proficient legal English as a professional. And improving English will be their life-long project.
By "motivation" I do not mean presenting a language as a "tool," that is how the anglo-saxon think about language. For me languages are portals to other cultures. I motivate students partly by introducing them to a small body of English literature, something their schools never did. Teaching them songs, poems by William Blake or Lord Tennyson. I taught them how to read English blank verse which is the gate to Shakespeare, and give them something to memorize.
This may appear somewhat high-brow but it proved fresh to students who had spent a life time learning grammar; most of them like my materials.
3. I attempt to explain grammar and vocabs as logically as possible. My knowledge of classical language (Greek) and French allow me to explain tenses better than contemporary textbooks. Students are always interested in etymology and your French or Latin can help.
Making language appear logical is one of the best way to draw enthusiasm from students. The law students I taught were not linguistically brilliant. Most law students are those who hate both English and math; it is actually a common knowledge that people who come into the legal profession don't have many aptitudes. (this is pretty true in the U.S. too) But if you present your subject interesting enough, they will learn.
4. Yes. Thailand's English language education is obsolete, inefficient and time-wasting. It is that bad. In my generation, we all went to public schools. But Thai upper-middle classes are now sending their kids to international schools.
Unfortunately, the upper class of the future will be those who speak bilingual but having no profound grasp of either language. I think it is sad that the elite are giving up on the state's education system and choose the path that will produce future elite who will treat languages as tools rather than culturally important assets.
Will the country ever produce quality literature when the better-educated don't even know enough Thai anymore? We will see.
5. Most farang teachers don't have background in languages, not even as a student. They come from all walks of life and come to Thailand to pick up language-teaching jobs.
Thai always appreciate native speakers. But language teachers should be adequately passionate about language and be able to transmit the desire to learn to students. Most of you don't even know another European language and may not have any interest in literature.
6. It is true that Thais are quite lazy. But they are not stupid.
Thais are 40% of Chinese ethnicity, they are genetically pretty smart. Thai culture is a bit like France - that is pretty laid back.
The whole state is bureaucracy, most people dream of civil-servant jobs that employ you for life. Thai bureaucratic system is like a socialist state with in a capitalist state. So it is not one of your corporate-driven societies.
For Thai kids, English is just another subject like math or chemistry. They are chores that kids have to put up with until the final exam. My point is that, to stimulate them to learn English, you have to present English as, not only professionally, but also culturally important to them. I believe that teachers should have some knowledge about local cultures to make connection with students.
I doubt most farang teachers know much about Thai culture. Thais are both conservative and nationalistic (think about a country where 80% are the GOP Republicans). Thus, they'd super appreciate foreign folk who learn their culture or languages. But Thais know more about the world than you think they do.
Maybe it's you who know too little about the place in which you are working