I created a form using English as primary language and Gujarati(gu) as secondary. When I replaced the link with &lang=gu appended at the last, the link opens in Gujarati except some differences.
The difference I noticed:
All buttons (i.e. Next, Back, Submit) are in primary language.
Placeholder text is in primary language.
While loading the page some details are loaded in primary language and the page refreshes with the language specified in second url with problems already mentioned above.
I have one more problem with regarding forms: How can we interchange primary language to secondary and secondary language to primary from the multilingual menu on right side. It seems that once you add a language in secondary languages list, you cannot add or update the primary language with that secondary language! This is my main problem!
Microsoft crew needs to make the available secondary language works correctly. I assume as the identifier language available doesn't mean they have the translated words of every component. It is cumbersome for whom expecting Microsoft form to work perfectly, and the user already scratched the primary language form.
If you have time, I suggest changing your form platform. Or you can ask your administrator to inquire about those problems. Lucky me, somehow my form works accordingly. Although, I clone the form and then open it with "the trick" has the same problem. And sometimes, the branching menu is disappeared.
Unfortunately, you cannot easily switch between primary and secondary language. I figured it out before asking the question. Cheers bro! Hope Microsoft could improve their Ms Form for us or institution who have paid for it.