Update categories.php

Corrected errors
Corrected grammatical/logic errors

Some sentences were not the Italian translation of the English ones
Corrected grammatical/logic errors
Corrected grammatical/logic errors
Corrected grammatical/logic errors
Corrected grammatical/logic errors
Update posts.php
Corrected grammatical/logic errors
Merge pull request #608 from edocod/master

Corrected and reviewed the Italian translation