content: Note that language identifier in file name must be lower case

[jmooring]

Please do not submit an AI PR. This is part of a larger issue, #3496.

Language identifiers must be lower case (e.g., content/foo.en-us.md instead of content/foo.en-US.md. See gohugoio/hugo#12829.

[jmooring]

This is a component of #3496.