Testing in my local wiki running trunk:
Notice: Undefined variable: wgResourceLoader in \UploadWizard\SpecialUploadWizard.php on line 60
Notice: Undefined variable: wgLanguageCode in \UploadWizard\SpecialUploadWizard.php on line 77
There is no such variable as $wgResourceLoader, so use something like class_exists( 'ResourceLoader' ). $wgLanguageCode is simply not being globaled.
$wgResourceLoader
class_exists( 'ResourceLoader' )
$wgLanguageCode
global
fixed both issues in r76424