gimp 2.2.12 multilingual does not start up if configured in this non-default directory structure.
Directory with GIMPPortable.exe (PortableApps, for instance) +\Apps\ +\GIMPPortable\ +\GIMP\ +\gtk\ +\Data\ +\GIMPPortable\ +\settings\
Following GIMPPortableSource\readme.txt this configuration should work. I get an obviously incomplete information message saying: 'was not found. Please check your configuration'