BUG Default Member.Locale now chooses a better default value when i18n.locale is not a valid translation

This will resolve issues in cases where the site locale may be assigned a value that does not have an explicit translation. E.g. if the locale is en_NZ (and it's appropriate for this to be the assigned locale), Afrikaans will no longer be the default selected locale when creating members. Now en_US is chosen as a better fallback default.

This is a minor ease of use fix that means fewer CMS users can be accidentally created in Afrikaans within NZ based sites.

Test cases included.