The mother tongue of most people in Hong Kong is Cantonese. We use it everyday : at home, in the street, in the shops, ....
Teaching involves communication : and communication is most effective in one's mother tongue.
Hong Kong had been under British Rule for 150 years (Kowloon and New Territories for 100 years). And Britain required that all schools, except those registered as "Chinese Schools", use English as the medium of instruction.
Because of this policy, and because jobs in government favored graduates from English Schools, most schools in Hong Kong were English Schools. Only a few were Chinese Schools.
I was in a "Modern School" (school that emphasized practical subjects) during Form 1 to Form 3. You may not believe it, the principal of that school was a Canadian and he was a noble person with a heart to education, that he prescribed that certain textbooks be in English, but the teachers were free to use Cantonese.
Let me digress here. The Maths textbooks there were "General Mathematics, Vol. 1 to 3, by J.B. Channon & A. McLeish Smith, published by Longmans". The textbook in Commercial course was "Office Practice, by W.F. Gregory, L. Jean Cameron, published by Ryerson Press of Toronto".
"Office Practice" was an excellent text-book. This first chapter of which is "Attitudes, Habits, Personality, Character" - those qualities and attitudes cherished in the business world. If you can locate that book in a library, I recommend you read that Chapter. It is excellently written.
Thus we had the best textbooks currently available, but the teachers used Cantonese, a language we understand.
I changed to an English School for Form 4 to Form 7. Again, I had access to the best textbooks in the Western World. But unfortunately, that School was a Government School, and teachers used English throughout.
I could understand only a small percentage of what the teacher said. Probably about 30%. But the textbooks were good, and good reference books were available, hence with the help of a dictionary, I can manage, even though I understood little during classes.
When I was in the University (University of Hong Kong). Most books in the Library are English. But I was not handicapped, I can understand English books, because I had been using English books from Form 1 to Form 7.
Hence I would suggest to the Education Department of Hong Kong : Use English textbooks, but freely allow the teachers to explain in Cantonese.
There may be good Chinese Textbooks, but if one is to communicate with workers around the world, whether it be in Science or Engineering, one must know English terms. Thus English Textbooks are a MUST. But students may freely read Chinese textbooks as reference books - indeed China published a lot of good textbooks and they are priced low.
USE ENGLISH TEXTBOOKS, BUT USE CANTONESE, OUR MOTHER TONGUE, AS THE MEDIUM OF INSTRUCTION.