Choose language block - flags instead of dropdown
concrete/blocks/switch_language/view.php to application/blocks/switch_language/templates/name_of_custom_template.php
Then make adjustments to that file, you will probably need to clear the cache for it to show up in the Custom Templates dropdown.
The other thing to note is that there is already a flags Custom Template built into C5.7 so you don't really need to do this anymore, you just need to go to your Switch Language block on the front end of your site, click Design & Custom Template from the menu, click the Gear icon from the edit bar and select the Custom Template from the dropdown.
If you are using Firefox get the Firebug plugin and check the Console there.
Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, checkhttp://xhr.spec.whatwg.org/.
The specified value "!" does not conform to the required format. The format is "#rrggbb" where rr, gg, bb are two-digit hexadecimal numbers.
That is what i get when i click Design & Custom Template in edit mode, but nothing more when clicking gear icon.