Adding cookie via RequestHeader

[adrift73]

I'd expect this RequestHeader to modify %{HTTP_COOKIE}.

RequestHeader add "Cookie" "locale=en-US"

But when I read the value of %{HTTP_COOKIE} right after RequestHeader it does not contain the locale cookie:

RequestHeader add "Cookie" "locale=en-US"
RewriteRule .* - [E=INFO_HTTP_COOKIE:%{HTTP_COOKIE}]

INFO_HTTP_COOKIE is empty.

I am using Apache 2.2.