Computer languages for my Website?
 

I'm building a video website, for which I’m using Python, as server-side language. What do I use for making the User Interface and the "Video Player"? I'm not sure HTML will give me the look for the website.

You need to build it in the standard language so HTML.
Look into HTML5 and see its new videos media enhancements tags.
Using CSS3 you can install a cool grid pattern system that can style all your media tags.

Also use PHP as a boost - like if you're using a CMS - otherwise just stay standard and you dont need to go complicated.