Hello, Codeforces!
Sometimes codeforces sets wrong languages when you test it. Sometimes even changing it does not work.
I was experimenting with C++11 and C++11 kept the default language for a long time!
It has been discussed in this blog Default Submission Language. Yes, I know it is 3 months ago, but it still happening sometimes.
After Installing this script, go to the submission page and you will see 'Save Option'. Note that it uses cookies to save your option.
Installation:
Tampermonkey or Violentmonkey or any other open source userscript manager is required. The source is available in Greasy Fork: Preferred Submission Language
Tampermonkey extension: Tampermonkey
Violentmonkey extension: Chrome | Mozilla Firefox | Opera