I would recommend you to write a pre chapter in which you explain that the original author isn’t active on Wattpad anymore. That you tried for a long period to contact him but he isn’t active anymore (or right now) and if it’s a problem that you have translated the story and the original author does not like that you can always delete the story then. I know that for some people ist better to have a story translated into their own language and I totally understand your statement. So yeah just go ahead, translate the story and see how it goes. Because the stories are a few years old and the author isn’t updating or using his account for a long time, I don’t think that it’s possible to contact him