The HTML tag (the outer-most tag of a TWiki page) has the lang attribute (and xml:lang attribute) whose value is from %LANG%. It's derived from {Site}{Locale} but you need to replace an underscore (_) with an hyphen (-) to be compliant with
RFC:4646 (which obsoletes
RFC:3066, which obsoletes
RFC:1766), which is referred to by the HTML 4.01 and XML standards.
There might be a situation where {Site}{Locale} derived value is not right. To cope with that, {Site}{Lang} is introduced to specify the %LANG% value explicitly.
<html ... lang="en_US" ... >
instead of
<html ... lang="en-US" ... >
confuses browser in some cases, and disables spell checking in a text area.
--
TWiki:Main/HideyoImazu - 2013-05-13