Hi,
They are translatable since HikaShop 5.1.0, but there is no interface for it.
You can translate them the same way you can translate some of the settings of the HikaShop configuration like the "product SEF name" and "category SEF name".
So for example, you can add the translation override:
MY_IMAGE_ALT="My image is great"
and then, instead of adding "My image is great" to the image description input in the image popup, you can add "MY_IMAGE_ALT" and HikaShop will automatically translate it with the override for each language on the website.
The main problem we have with this is more of a structural issue. Since the image name field is already in a popup, we don't want to have a popup in that popup for the translations. And there is not much space for flags on the image itself. So it's more of a UX issue.
I guess we could have tabs inside the popup, like we have on the payment / shipping method edit form.
So far, no one else talked about doing this, so I'm also not sure if it's worth it for us to spend several hours working on this...