There was also an update to be able to read the preferred locale from the user’s browser. If you go to Custom Fields and set the Preferred Locale field as publicly updatable, there’s no other action you need to take. Mautic will recognize the locale from the browser, and if you have the translation created properly on the email you’ll be able to use just one email in the campaign.
The Mautic documentation on translations seems to be a bit lacking, but here’s what I could find:
https://docs.mautic.org/en/5.x/channels/emails.html#translations
Forms can’t be translated (unless that’s an update in 5.x), but you could use dynamic web content on your page to show one form if the preferred locale is English and a different form for German (you can use any other languages too, of course).
Hope that helps!