InfoPath DIP today does NOT support MUI, it's possible to quirk around with xml files and translating manually, but in a scenario where SharePoint already supports MUI on fields/content-types etc. this creates more hassle than you want. Whatever you do with the DIP, make it easier to support MUI, preferrably by using resources already in SharePoint.
Thank you for your suggestion! We’ve passed this along to the engineering team and we’ll keep you posted via the blog (http://blogs.office.com) if and when we implement this feature.
Valy Greavu commented
Jakob it is really hard to translate all fields and labels from a form. Think about programming! You have to translate ”manually” the expressions, messages, etc.
You can study my version of multi-language forms: https://www.youtube.com/watch?v=g1c2td7ow_M