WPLANG interferes with the interface language whilst installing

[johnbillion]

Steps to reproduce:

• Set a language in the WPLANG constant in wp-config.php and ensure you have the language pack installed for that language
• Go through the installation procedure, but select a different language on the language selection screen.
• Note that the language in WPLANG is used in the interface, instead of the language you selected.

I've narrowed this down to the load_textdomain() calls in wp_install_load_language() which aren't having any effect.

[johnbillion]

Fixed in [29059]