Python/Django or C#/ASP.NET for web development?
Almost all the well known frameworks and languages can scale.  
It doesn't really matter which one you use.  Its about how well you structure the code that matters most. 
On a personal level it is always good to know more than one language.
But, you can create perfectly scalable Python, PHP, .NET applications.  The quality of the code is the first place scalability will fall down not the language.
I am a .NET developer. I have no experience on Python. Which platform is more scalable? Which platform is more suitable for large-size high-traffic web sites? If you have any experience about scalability on these platforms, please inform us.
Thank you.