How can I install Gimp 2.8's help documentation on Ubuntu 12.04?
Most likely, the PPA does not contain the required package for the documentation, or GIMP's documentation has not been made ready yet for 2.8.
In any case, try
apt-cache search gimp-help
and install the appropriate package for your language.
I'm working on it.
There are some some compiling issues.
Please wait.
Thorsten Stettin
https://launchpad.net/~otto-kesselgulasch
It looks good.
Please wait for publishing on my Gimp PPA.