How can i add flags in a language selector item in HTML and pure CSS?

Question

Disclaimer: I am not allowed to do this in any CSS libraries, only pure CSS

I want to create a language switcher button like in the picture:

see here

But the problem is I can not add flags to the selector, it does not show up? Here is my code, and it only displays text not flags.

<select>
      <option><img src="US.png" /> English EN</option>
      <option><img src="TR.png" /> Turkish TR</option>
</select>