It is not possible to have a data race with environment variables across multiple processes. Every process has its own copy of environment variables (in fact they have their own copy of the entire environment).
I'm not sure what data race is possible across processes with locales, that's too vague of a claim to make.
One type of locales I know are the LC_ env vars. So there the "ENV is a copy" applies too.
Another would be to read and write into locale files, such as JSON. But then the ame applies as with any database or IO: this is inherently race-condition-prone and that is by design.
Maybe grandparent is thinking about locales in many web frameworks, that is some global var which should not be shared across users. So that if you set `Locale.current = "EN_GB"` that applies for any (email)notifications, errors, files, responses or such, being sent out during that request/response and during any jobs that request/response may spawn. In e.g. Rails this "somewhat global var" is a Frankenstein, but works suprisingly stable, actually.
I'm not sure what data race is possible across processes with locales, that's too vague of a claim to make.