I'm migrating a python application from an ubuntu server with locale en_US.UTF-8 to a new debian server which comes with C.UTF-8 already set by default. I'm trying to understand if there would be any impact but couldn't find good resources on the internet to understand the difference between both.
I can confirm there is effect on different locales (C.UTF8 vs en_US.UTF8). I recently deployed one python program into a new  server, and it performed differently. The old and new servers are both Ubuntu 18 servers, and the only difference is the locale (C.UTF8 vs en_US.UTF8).  After setting the locale in new server as C.UTF8, they behave the same now.
It is easy to set the locale for a single application in Linux environment. You just need to add export LANG=C.UTF8; before your application. Assume you execute you application as python myprogram.py, then you type:
export LANG=C.UTF8; python myprogram.py
What is the difference between C.UTF-8 and en_US.UTF-8 locales?