For customers with multilingual sites it's a must to allow a/b testing for pages with translations.
The actual solution recommended by the HubSpot support is an evil hack around that undermines all developers efforts to provide easy to use multilingual solutions.
"The A / B test is attached to the "standard" language, in your case EN, not to both languages. If you want to run both languages as an A / B test, you need to create separate landing pages, one for EN and one for DE. I would suggest that I have an idea in the Idea forum to create what a feature will be available in the future. For such an idea to be implemented."
So please go for it and make it happen asap!