Differences between locale and Language

There is a system variable called “locale” with the following description:
“The user’s browser locale, such as en-US, en-CA, fr-FR, pt-BR, etc.”

There is also a “Language condition” building block that is “based on the user’s preferred language setting in their browser.”

I’ve had several occasions where greeting the user using locale and then using a hard-coded output via language condition created a mix of languages. I’ve tested it and both browser language and preferred language where identical.

The bug seems to be that these are not consistent.