Eso es discutible
 

Eso es discutible. How can I say in English?

That's debatable.
That's arguable.
That's disputable.

As Rusty says, "that's debatable" is probably the best / most common translation. Depending on context there are all kinds of ways to render it though:

That's up for debate.
That's up for discussion.
That's on the table.
That's not written in stone.
That's fair game.
That's questionable.

But again, context and what nuance you want to give it all matter.

Also with another connotations, it could be "dubious".

una persona de gustos muy discutibles — a person of very dubious tastes

Also, in the line of "that's debatable" from Rusty, "that's a matter of opinion".

Saludos Robin.

Use a moot point if you wish a direct translation.