How do I ask the author of a book in English if I can translate it into my native language? I want to translate an English book, but before translating it, I want to be sure if the author will permit me to do so.

I am not doing this for financial gain. Since it is a book about a new subject, my professor asked me to translate it, so that students in my country become better familiar with the topic.

Would you please provide me a good text to email it to the author?
In most cases books are published through a publisher and certain copyright laws apply that may be shared between the publisher and the author. Exactly where the right to decide on translations lie may vary. You therefore need to contact both the author and the publisher to look into what might apply and to what extent they are interested in such translation.  My suspicion is that it will not be as easy as just getting permission to translate. When a book (or any publication) is translated, there will be a need to get a translation that properly represents the original content. A publisher may not be content with "anybody" translating the work, they may request some form of review etc. So, the bottom line is that you need to contact publisher and author to see what is possible and under what conditions. It is possible they accept translation into a different language for different reasons but they will most likely want to retain some form of copyright.