my $selected_dir = "$CFG->{admin_root_path}/templates/$CFG->{default_template_set}";
my $language = GT::Config->load("$selected_dir/language.txt", { create_ok => 0, inheritance => 1, local => 1, debug => $CFG->{debug_level}, header => <<HEADER });
# This file is auto-generated and contains a perl hash of your
# language variables for the '$selected_dir' template set.
# Generated: [localtime]
# vim:syn=perl:ts=4
HEADER

$language->{LANG_VAR} = 'La la la.';
$language->save();