When I translate a post, will its relationships be translated?

I translate the English page, with an ACF post object field. When the page with the ID attached translates, instead of automatically creating the relevant translation post for the ACF field, it ignores it, and in the logs I see a warnings saying:

There is no translated entry for custom post or media item with ID '2439' and language 'fr'

Do all translation posts need to be created first for this to work? Will it not create them on the fly if one doesn't already exist?

All the referenced entities must already exist (i.e. translated). When you translate a post, it only translates that post. It doesn't trigger the translation of all other entities it references.

So you have to translate the referenced post first, then the post that references it.

The same applies with categories/tags/featured images: you have to translate the referenced entity first, then the post that references it.