Asking about sufficiency of a document
It depends on what you are trying to get at. Either "can" or "could" are basically saying the same thing. They are asking if there is any possible situation in which an abstract is enough. That is, is there some set of circumstances that might or might not apply to the current situation when the abstract is enough?
Possibly you want to ask if an abstract is sufficient in this particular case. In that case, you want to say "Would an English abstract be enough?"