LINGUAS standards?

[robert lopez abq sc philips com]

Is there a standard for the environmental variable
LINGUAS?

I am in the USA and I sometimes have to set it to "en"
sometimes to "en_US" and sometime I have to unsetenv
it.

I see there is a a "en_GB" and wonder if I should use
it.

Why do different packages/programs etc have different
uses of it? Is there a standard for it's use?