better translatable messages (modified suggestions of Lukasz Daniel) + his polish translations

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/matrix@2977 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
using .hasSlot() is faster than checking for "compMatrix"

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/matrix@2978 edb9625f-4e0d-4859-8d74-9fd3b1da38cb