New translation: galician

[davide]

Created and attached Galician translation.

Please add it.

Also, it would be great if you changed the following line
in "roundcubemail/program/localization/index.inc":

'ga' => 'Galician',

by

'gl' => 'Galego',

Both ISO 639 and ISO 639-2 says that "gl" is the correct language code
for "galician", and "Galego" is the native name of the language

ISO 639 (obsolete)
​http://www.w3.org/WAI/ER/IG/ert/iso639.htm

ISO 639-2
​http://www.loc.gov/standards/iso639-2/php/code_list.php

Thank you very much!