Simple Certificate - Language fix

This fixes the following issues:

1) Adds output filtering (e.g. multilingual filter) when rendering the PDF certificate in the certificate, the tags and in the PDF document properties.

2) Fixes the keyword list in the PDF so that it don't end up with a duplicate string separated by a semi-colon (;).

3) Adds output filtering (e.g. multilingual filter) of download filenames of Issued Certificate in ODS, Excel and Text format.

4) Corrects some spelling typos and formatting issues in the English language file.

5) Replaced tabs with 4 spaces and removed trailing spaces.

Affected files include:

* locallib.php
* lang/en/simplecertificate.php

Signed-off-by: Michael Milette <michael.milette@instruxmedia.com>