If he NEVER responds to you... nor to his name, nor to any sudden loud noises.... ???? Or does he?
He is also being raised multi-lingual.... so, talking/speech is entirely DIFFERENT from 'hearing impairment.'
My kids are bi-lingual and my Daughter also learns other languages in school. They are fine. My son though was delayed in speech. No biggie. He is now the most talkative one in our family and very articulate and had very GOOD comprehension of both languages AND the other languages he picks up.... he can count in English, French, Spanish, Japanese, and Chinese, for example. And he knows/speaks words in all those languages too. My daughter as well.
Since BIRTH, my kids were spoken to in different languages. It is fine. They are now 4 and 8 years old. No problems. And they respond to people, no matter what language they are spoken to in.
Why does everyone think he is deaf? Does he ever turn his head toward you.... in response to calling his name? Or you asking him something? Or you making loud noises???? Or does he startle in reaction to LOUD or scary sounds???? Does he react to music???
He is already 18 months old... he MUST have had, well-baby check-ups by now... not only now. To me, a Doctor would have mentioned or noticed it if he were 'deaf' or had hearing problems... or you would have mentioned it to the Doctor, a long time ago at each of his well-baby visits and annual physical exams.
Babies, see the Doctor every 3 months, then at 12 months, 15 months, 18 months, and 24 months. So... did he? This is the usual check-up schedules.... for a baby.....