I18n message improvements: Fix typos; unify strings.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23388 57a11ea4-9604-0410-9ed3-97b8803252fd
First update of German translation.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23389 57a11ea4-9604-0410-9ed3-97b8803252fd