All twelve Swallows and Amazons novels have now been published in Simplified Chinese (the script used in mainland China) by Guizou People’s Publishing.
Here are their names in Chinese (characters and Roman alphabet), followed by a translation of the Chinese name, then the original english name, then the ISBN number:
燕子号和亚马逊号 Yanzihao he Yamaxunhao: "Swallow and Amazon" [referring to each boat, not their crews; 号 "hao" is a suffix after a boat name, a bit like "HMS" in English; 'Yamaxun' is a transliteration of 'Amazon'] (Swallows and Amazons) 978-7-221-099440-0;
燕子谷历险 Yanzigu Lixian: "Swallow Valley Adventure" (Swallowdale) 978-7-221-09943-3;
蟹岛寻宝 Xiedao Xunbao: "Crab Island Treasure" (Peter Duck) 978-7-221-09941-9;
向“北极”进发 Xiang "Beiji" Jinfa: "Marching to the 'North Pole'" [note "North Pole" in quote marks in Chinese title] (Winter Holiday) 978-7-221-09948-8;
布尔河畔的黑鸭子 Buer hepan de heiyazi: "Black Ducks of the Bure River" [coots, I presume] (Coot Club) 978-7-221-10501-1;
鸽子邮差 Gezi youchai: "Pigeon Post" (Pigeon Post) 978-7-221-10503-5;
雾海迷航 Wuhai Mihang: "Lost Amid the Fog" (We Didn’t Mean to Go to Sea) 978-7-221-10507-3;
秘密水域 Mimi Shuiyu "Secret Waters" [bodies of water] (Secret Water) 978-7-221-10508-0;
侦探六人组 Zhentan liurenzu: "The Six Detectives" (The Big Six) 978-7-221-10504-2;
逃离龙虎岛 Taoli longhudao: "Escape from Dragon Island' ['from' is implied by context] (Missee Lee) 978-7-221-10505-9;
皮克特人和殉难者 Piketeren he xunnanzhe "The Picts and the Martyrs" (The Picts and the Martyrs) 978-7-221-10502-8;
保卫白嘴潜鸟 Baowei baizui qianniao "Defending the Great Northern Divers" [the birds are called 'white mouthed loons/divers' in Chinese] (Great Northern?) 978-7-221-10506-6.
Thanks to Arthur Ransome Trust for the news of publication and Google Translate.