I found that if I wanted to add a culture that is not listed in the default .NET culture list, the CMS would not allow it and give me a ‘Can't create new culture. The culture code is not valid.’ error. In this case it was en-HK (English Hong Kong).

Any unused culture could be used but for consistencies sake it's quite simple to register a new custom culture which is persisted to the machine (using admin priviledges). MSDN has a code sample here (http://msdn.microsoft.com/en-us/library/ms172469.aspx), it's only a few lines of code if you're using an exisiting culture (I used en-US and changed the region to HK).

The only problem i found is you need to register the custom culture on all the webservers, but i think it's only a minor stumbling block, but would be ideal if this was all in the CMS. I haven't found any other issues with doing this but if anyone has a better solution please let me know.