The redirecting “drop-down” (select element) part of your question can be answered by looking at this fiddle that someone made for a different question:
https://jsfiddle.net/jalbertbowdenii/2f9wvyqm/
As far as getting the flags to appear in the option elements of the select element, the only way to get it to work cross-browser (that I know of), would be to find some kind of fancy font-icons to use.
Otherwise, for best compatibility, I would suggest faking a select element with some jQuery or a jQuery Library.
The topic of images in option elements, and many of the available options, is covered thoroughly here – including one example of a library with a flag-bearing language selector.