YD Setup Locale
It will automatically setup the right XML header lang attribute + the internal WordPress $locale variable that is used to load the right language files for i18n and l10n (.po, .mo).
Your blog URL structure should always begin with an official two-letter langage code, for example:
/en/… for english
/es/… for Spanish
/ja/… for Japanese
/fr/… for French
Pages that don’t comply to this URL scheme will still get your blog’s default locale setting, as configured in your wp-config.php file.