Updated and added 'pl' language files

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rename : plugins/blog/languages/en.lang.php => plugins/blog/languages/pl.lang.php
rename : plugins/box/dashboard/languages/en.lang.php => plugins/box/dashboard/languages/pl.lang.php
rename : plugins/box/emails/languages/en.lang.php => plugins/box/emails/languages/pl.lang.php
Adding translation of the word 'Continue' in the file install.php and the related update language files 'en' and 'pl'.
Merge pull request #234 from mwidz/dev

Updated language files and adding translations.