Include GlotPress language database

Check `class-languages.php` method `is_rtl` and whether we can use the database of language information included in GlotPress ([`class GP_Locales`](http://svn.glotpress.org/trunk/locales/locales.php)).

Start with class-languages.php and its is_rtl method, then compare the existing language information with GlotPress's GP_Locales database at the linked locales.php source. Determine whether that database can be used for the language-direction data, and consider the work complete when the issue's proposed reuse or integration is clearly supported by the implementation.

Written by the indexing model from the issue text.