It's used in the Hugo-internal template "opengraph":
2fa70c9344/tpl/tplimpl/template_embedded.go (L205)
Added new keywords HUGO_WITH_LOCALE (defaults to nil) and HUGO_LOCALE.
With HUGO_WITH_LOCALE non-nil, and HUGO_LOCALE nil, the locale is
auto-detected from the system env vars LANGUAGE, LC_ALL or LANG.
User can also choose to manually specify the locale instead using
HUGO_LOCALE using a "language_TERRITORY" type string.
Note that the auto-detected/manually specified locale will be exported
to the "locale" front-matter only if HUGO_WITH_LOCALE is set to a
non-nil value (like t).
https://github.com/kaushalmodi/ox-hugo/issues/122
Kaushal Modi