For anyone else who runs into this issue, a quick, temporary fix is to hack the LanguageFunctions.php file. For all three functions in there, you'll need to wrap each with a check to make sure the functions haven't already been declared. For example, the first function:
if (!function_exists ('__')){
function __($text, $textdomain = null, $context = '') {
...snip...
}
}
My best guess is that this not recommended as a permanent solution since it may cause issues with WordPress. And of course, ProcessWire upgrades will overwrite the hack.
Best solution? Don't use Wordpress.
For reference, here's Ryan's awesome Blog Profile for ProcessWire.
http://modules.processwire.com/modules/blog-profile/
Hani's post in Wordpress in subdirectory error was marked as the answer