This is available as
Locale\get_native in the www repository.
Retrieve the locale being used by libc functions for the current thread
namespace HH\Lib\Locale; function get_native(): Locale;
In general, we discourage this: it can be surprising that it changes the
behavior of many libc functions, like
sprintf('%f'), and error messages
from native code may be translated.
For web applications, that's likely unwanted - we recommend frameworks add the concept of a 'viewer locale', and explicitly pass it to the relevant string functions instead.