Thanks for the report! The web team was able to identify an issue with one of the webservers which caused this to happen. We have pulled the offending server offline. If anyone else encounters this issue, please let us know.
We believe that this issue is resolved as of September 2020. If there is no Accept-Lang header, the website should automatically add the en-us portion of the path. Can you confirm?
Thanks for the report! The web team was able to identify an issue with one of the webservers which caused this to happen. We have pulled the offending server offline. If anyone else encounters this issue, please let us know.