I'm not sure how to get around that - it's an annoying problem, as it aborts installation of everything. One workaround is to get the Bundle-Lingua-Stem.zip archive from http://theoryx5.uwinnipeg.ca/ppms/bundles/, which contains the Lingua-Stem ppm package and all of it's dependencies. Unpack this to some directory, add this directory to your list of ppm repositories as

ppm rep add temp_repository  
       file://C:/Path/to/current/directory
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and then run something like the following in that directory:

use strict;
opendir(my $dir, '.');
my @ppds = grep {/\.ppd$/} readdir $dir;
closedir $dir;
foreach my $p (@ppds) {
   my @args = ('ppm', 'install', $p, '--nodeps');
   print "@args\n";
   system (@args);
}
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This will use ppm to install all the packages without worrying about the dependencies; it'll fail on the Lingua-Stem-Snowball-Da package for the reason you found, but at least the others will get installed.