in 1714 would the most common second language in Japan be Dutch, Chinese or something else?

It would have likely been Chinese.

The time period implies that the people of high society would still consider mastery of Chinese to be a sign of an educated person, an elite.

Meanwhile, contact with Europeans was heavily restricted and even then, mostly to traders and officials that would directly interact with those traders. So the number of people who knew Dutch or Portuguese well would have been few indeed.

Other exposure to foriegners would have probably been with Christian missionaries, few and far in between because of persecutions and further restrictions on movement and who they could meet.

But even assuming they met with the common people at all, the missionaries would likely use Japanese instead of trying to teach or find people that knew their European language. They would be trying to find converts and being able to understand Japanese culture and language would help to endear them to the locals.