Describe the bug
Langauge list in newsapi/const.py contains invalid language and 2 of them don't follow ISO-639-1 standard.

To Reproduce
Try to use 'se' country code with /everything endpoint, it won't give results and isn't available on NewsAPI documentation.

'cn' and 'en-US' aren't ISO-639-1 codes

Desktop (please complete the following information):

• OS: MacOS, Arch Linux
• Browser: Chrome, VSCode (Python 3.10, iso639 library)