You can test it with your own multilanguage "classic cars for sale - classifieds site".
When you first select "Spanish" as language and then afterwards go to Toolset > Post Forms > "Submit new listing" (edit)
(first switch to Spanish, then go to this direct link: hidden link)
Then there is no product selected (see screenshot).
If you then select the Spanish product-translation in the dropdown and hit save, and then switch language back to English
=> Bug 1: it opens "create new form" (instead of same edit-form in English)
=> Bug 2: (much more critical): if you after switching to English go back to the form (hidden link) then the English product is no longer linked!
In short, you cannot correctly link a multilanguage (WPML) product to a 'Toolset Forms Commerce' edit form.
Thanks for writing back and for sharing further details.
During troubleshooting on my own test website with a similar setup, I was able to reproduce both these issues.
Appreciate you brought this forward and I've passed on these findings to the concerned team. I'll keep you updated with the progress through this ticket.
Note: During testing, I was also able to confirm that even if no product is shown selected in the form's settings after the language switch, this doesn't affect the actual checkout process on the front-end. Visitors will still be able to submit the form and the correct product will be added into the cart, based on their active language.