This might seem just like a easy and trivial question, but I am while using following tag inside a WordPress theme header file:

<html <?php language_attributes(); ?>>

That is outputting:

<html dir="ltr" lang="en-US">

If only to alter the lang attribute to "en-GB" as my blog and also the language posts are designed in are British British (en-GB) however i aren't able to find where this parameter is occur the WordPress admin configurations, there is not something for this within the wp_options database table either, which leaves me to think setting the lang value should be some kind of dark art?

Define the word what attribute using:

define ('WPLANG', 'en-GB');

You will get a lot of it at http://codex.wordpress.org/Installing_WordPress_in_Your_Language

You can include and apply filters in wordpress. try that ...