Why is it “absolutely ineffective” to teach in the student’s native language? If you’re learning French grammar, whether you’re learning it in English or French, what you’re learning is French grammar. It might take more words to explain the same concepts in a different language, but you’re still learning. So it’s less efficient, but calling it “absolutely ineffective” seems like hyperbole.
I think the most important thing is efficiency. For beginners, it’s more efficient to teach in their native language. Otherwise they won’t get most things and it’ll take more time for them to understand what’s being explained. You’re just adding a communication barrier. For more advanced students, it’s more efficient to teach using the target language, because explaining the grammar of a language using another language requires more words.
It’s usually quite easy to gauge a student’s level. I’m not going to teach in Arabic if the student stops me every second sentence. But I expect more advanced students to be able to understand explanations in their target language.
I prefer having a teacher who speaks one of my native languages (which usually means English), just to have it to fall back on. When the teacher only speaks my target language, explaining words I don’t understand can take too long, and that hurdle is unnecessary.
I am curious how often it has happened to you that you asked a teacher to teach in your target language and they continued to teach in English.
I think there’s often pressure on students to start speaking in the target language as soon as possible, and usually that means immediately. It can be quite discouraging. I hated language-learning back in high school because of this approach. I found it overwhelming. I think maybe some teachers like to make sure you have the option to learn in your native language and that they’re not judging you for it. But it's weird if they continued to teach in English if you had asked them not to.