According to the blog Thrill List;
The sequence is even more challenging because we're never given the dialogue MacGuffin. Heisserer says Villeneuve debated whether or not to include subtitles for the line. The director opted to revel in the mystery. The writer wasn't as keen on keeping it a secret, and was happy to divulge. As he told the audience at Fantastic Fest, the line translates to: "In war there are no winners, only widows."
"I worked so hard on the dialogue in Mandarin for Denis," Heisserer wrote on Reddit. "Spent weeks crafting the lines that he finally approved! And then that scoundrel goes and doesn't use subtitles in that scene. I guess there's something to be said there about the nature of language. And I love Denis. But he's also a mischievous fox."
Found at: https://www.thrillist.com/entertainment/nation/arrival-chinese-line-ending