Adding a new language: Language I need (welsh/cymru/cy) is not available.

Hi,

This is driving me nuts, and I'm sure it's a really simple fix but I can't
find an answer in the docs or on the archives of the mailing list.

I would like to be able to offer a Welsh version of the website I am
developing. Wales is down in ISO639 as CY, but there is no 'Wales', 'Welsh',
'Cymru' or 'CY' options anywhere in the 'Available Languages' on my 'Manage
Languages' admin page.

Actually the question has been asked before:
See http://article.gmane.org/gmane.comp.cms.jahia.devel/3462

But the response seemed to suggest that all languages should be displayed in
the 'Manage Languages -> Available Languages' admin area:
See http://article.gmane.org/gmane.comp.cms.jahia.devel/3463

This isn't the case for me (is it for you?). 

Can you point me to some documentation which tells me how I can add the
language (one of the oldest in Europe for those who are interested in such
things!)?

Many thanks,

Anton.