Properly translate select attributes in shop API

[dnna]

For select attributes, the translations set in the attribute configuration aren't used, but instead, the UUIDs are shown with no way for the user to fetch the real string. See below screenshot for an example:

This PR fixes this by specifically handling this case and retrieving the correct string for use in the API instead of the UUID.

[mamazu]

Thanks you for your contribution. Would you mind showing an example on how the API response will look after the PR?

Second I don't think replacing the UUID strings is a good idea. People using the API will rely on that. I'd rather suggest introducing them in a separate property.

Other than that it looks good. Would you mind changing the tests and documentation to reflect your changes? If you need any help feel free to ask. :)