Checks for privacy and security sections don't work if lang is set to en-US

[jnurthen]

Description of problem

Checks for privacy and security sections don't work if lang is set to en-US. Changing the Document language to en correctly shows the warnings but having the lang set to en-US does not.

Document languages are recommended to be as specific as possible to en-US is correct.

URL to affected spec or repo:

https://w3c.github.io/aria/

What happened (e.g., it crashed)?:

Gives console warning Missing localization strings for en-US.

Expected behavior (e.g., it shouldn't crash):

Should complain about no separate security / privacy sections

[xfq]

The reason may be because ReSpecs's language matching method is relatively simple. Ideally, the algorithm in BCP 47 should be used.

See https://github.com/wooorm/bcp-47-match for an example.