Forgive me but in the interest of learning we must have correct understanding of how to learn/teach.

After reading and finding your article a little odd, I looked up the reference you give.

I think "the target language" means "the language being learned" (not the First language -- in your word "ภาษาที่ 1"). So, if Thai sudents are learning English, then teachers should use English to (teach English) to students. I think you should consider editing your article.

(PS. Please also note the original title: 'Essential tips teachers of modern languages'.)

[Davinia Hardwick, formerly a British Council English language assistant and now Head of French at a UK school, gives us her tips: (in teaching French in UK)

Use the target language in lessons

The more you expose your students to the target language, the better. Occasionally, speaking English may be necessary, but a lot of the time, it is not. Immersing your students in the target language helps them use it more independently and this can lead to increased confidence and better vocabulary.

Students enjoy listening to 'real people' speaking the target language, so get your colleagues involved. If a teacher with some knowledge of the target language comes into your classroom, involve them in the immersive experience. If they are struggling, the students can help them improve their language skills. If they already speak to a high level, it shows the communicative value of language skills. Encourage students and staff to use the language, even if they make mistakes, and emphasise that communication is the key.]