Helping us is very easy. Get a working copy of the code and join our mailing list. Feel free to make changes on your working copy and feel free to tell us where you'd like to help.
Create a local working copy of gimp-help-2 for yourself.
$ git clone git://git.gnome.org/gimp-help-2
Join our mailinglist at: https://mail.gnome.org/mailman/listinfo/gimp-docs-list
You can check the GNOME bug tracker in order to contribute patches: https://bugzilla.gnome.org/browse.cgi?product=GIMP-manual
To find out about the current progress of each translation use Damned Lies.