About the double quotation marks

Greek Learning Greek
1

I met it today for the first time. In my textbook, double quotations marks are „“ (like in German). Is this a common way?
I searched it and found that many people use «» or just “”, rather than 99-66 marks.

2

Because the author of my textbook is a professor mainly focusing on German philosophy, and greatly affected by German language, I’m not sure if that is his habit or a common method.

3

You can use double quotation marks or single, or nothing. It’s only a modern convention to use quotation marks at all.