Extensions can register message groups for Translate (CentralNotice does so), but that might be the way to go or not.

Hi Friends, we have applied for a Grant for this project (see here)

If you think this project should be selected for a Project Grant. Please add your name and rationale for endorsing this project. Other constructive feedback is welcome on the discussion page.

I just went to add my name to the "endorse" section (sorry for the long delay), but I see that it was marked "ineligible" a few weeks ago:

https://meta.wikimedia.org/w/index.php?title=Grants:Project/Wikifab/Translate_compatible_with_Page_Form&diff=16534856&oldid=16528529

That's too bad. But getting this accomplished may not require funding - or at least, not as much as you asked for. This may be easier to do directly within Page Forms, rather than via a new extension, as you had planned. More information may be helpful here. You said that you want fields to be definable as "translateable". What exactly does that mean, in terms of the syntax? And which field types would this cover - just "textarea" and "text"?

Yes, endeed, it was marked "ineligible", because no wikimedia projects is using pageForm ... :/

But we will work on it even if we don't ge the funding (maybe a simpler version as suggested)

To define a field "translatable", we could use this syntax for exemple :
{{{field|description|input type=textarea|translatable}}}

For the user who is using page form te edit a content, i will see no difference, but when the page will be marked "ready to be translated", then the pageForm extension should add the wikimedia "<translate>" tags around the field value (this will not be visible for the end-user)
This simple feature could make a first step very valuable, and translators could then use the standard wikimedia translate interface.

@Pyro853: Hi! This task has been assigned to you a while ago. Could you maybe share an update? Do you still plan to work on this task?
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If this task is not resolved and only if you do not plan to work on this task anymore: Please consider removing yourself as assignee (via Add Action... → Assign / Claim in the dropdown menu): That would allow others to work on this (in theory), as others won't think that someone is already working on this. Thanks! :)