Most multi language sites show the locale in the URL. For example: https://www.stackoverflow.com/en. With Craft the issue seems to be that the default language seems to also work without the locale in the URL. This is also causing indexing issues with Google. I currently have languages setup and both have their own base URL which consists out of the same hostname but end with a different locale.
But Craft does not automatically redirect users from https://www.stackoverflow.com to https://www.stackoverflow.com/en. Even though this is the default. And both https://www.stackoverflow.com/en/about and https://www.stackoverflow.com/about seem to be valid URL's.
Would it be better if I would remove the locale in the URL from the default language? It's a little bit weird to have it setup this way since it will not have a consistent URL structure for all languages but it might fix my indexing issues.