1 - As i've searched, the OPTION element is not cross-browser stylable correct? It only assumes background-color and color. I cannot add padding for example.
2 - It's only stylable using bootstrap, which means that <option> would be a LI inside UL. If that's the case, when will it be implemented?
1. That is correct, the select options rendering is completely dependent on the user's browsers and OS and the built in styling they have for it. So viewing the same select on Macs would look different in Windows.
2. There is styling to some extent with bootstrap however it will still look different from to browser. hidden link
3. No if you write this is plain HTML then it won't show up for string translations. If this was a CRED shortcode then it should show up for translation.