Only fields with the 'translate field' option enabled will be translated, this also works with the machine translation services for Microsoft and Google. But indeed, you can only do this for fields on the Content tab of the Pages application and for page file attachments.
If you're looking for a way to translate the caption/description field label instead of the actual data entered, you can do so. Both fields are localizable and in the localization application you can translate and insert strings with a single click using the machine translation service of your choice. An example is in Translating localization strings using machine translation.