Do the French temporary language pack have these strings yet, or might it just be fall back to English when those are not available?

If only it was that simple. What is being displayed here is the value of a field. So it can be whatever you want it to be - it just can't be translated
<?php echo $this->escape($item->stage_title); ?>

I have already discussed that with @bembelimen

There is a first patch here to test:
#21506

Then some more will come. Basically it will work like custom admin menu items where the title is displayed through Text::_(). Some strings obviously already exist (JPUBLISHED, JTRASHED, JUNPUBLISHED, JARCHIVED) other will come from workflow ini, custom ones will need users to create strings for their custom workflows.

Do the French temporary language pack have these strings yet, or might it just be fall back to English when those are not available?

we do not have French or German or Persian temporary languages.
We are using for now the packs from 3.x

So, the strings above already exist in these packs, but not the new strings from com_workflow

And yes, new ones will be replaced for the moment (after future patches) by English when not present in these lang packs.
Below example for the action Publish, etc.

Closed as duplicate