How to retrieve blog language without any region (locale) attached?

I’m posting this at the request of one of our Discord users, who doesn’t use WPSE:

Stack Overflow / Stack Exchange is kind of a bizarre place. Why is
that question voted “-2”? The question is basically asking more or
less the same thing that multiple WordPress tickets are asking for:

https://core.trac.wordpress.org/ticket/48152

https://core.trac.wordpress.org/ticket/50183

Also the W3C itself seems to recommend the same thing (avoiding the
region in most cases):

https://www.w3.org/International/articles/language-tags/

https://www.w3.org/International/questions/qa-choosing-language-tags

But somehow the question offended the WordPress techbros, so it got
voted -2

To answer the question: WordPress does sort of have a way of getting
this, but it’s awfully obscure:

https://api.wordpress.org/translations/core/1.0/

If you pretty-print that, the entries look like this:

{
    "language": "en_GB",
    "version": "6.4.1",
    "updated": "2023-11-24 12:26:36",
    "english_name": "English (UK)",
    "native_name": "English (UK)",
    "package": "https://downloads.wordpress.org/translation/core/6.4.1/en_GB.zip",
    "iso": {
        "1": "en",
        "2": "eng",
        "3": "eng"
    },
    "strings": {
        "continue": "Continue"
    }
},

The “iso” property contains the 2-letter and/or 3-letter language
codes, without any region code. (I’m not sure why “eng” is there
twice.) WordPress itself uses this for the lang=”…” attribute in the
language dropdown on the page wp-admin/options-general.php Also
“en_US” is a special case (since it has no translation) and is just
hard-coded as lang=”en”:

https://github.com/WordPress/WordPress/blob/6.4.1/wp-includes/l10n.php#L1616