The default language is the language of the browser and English is the fall-back. Of course it will be better if we had languages without countries.
To customize the css and logo, you just have to edit the index.html or use your own.
This is exactly the problem that should be solved with issue5443
Except it requires to fetch one file before being able to fetch and render the HTML/CSS.
Instead of forking index.html, it is better to serve another html page. This could be done by fixing the TODO.