Currently the way you would do that is by using the short version in the custom field definition. This will be the programatic value that is persisted. For example:
US DEThen in your message bundle (managed by the theme), you can translate those values into a more human readable option.
US=UNITED STATES DE=GERMANYIf we do not find a localized key, we will just use the key as the value which would then render as <option value="US">US</option>.