Vietnam diplomatic document signed by Vietnam Premier Pham Van Dong
Translation of Vietnamese government's diplomatic document (shown above) into English:
The Democratic Republic of Vietnam's Government agree to terms of China's public statement on 9-4-1958 about China's sea territory claim. The Democratic Republic of Vietnam Government respect it, and will direct all Agencies to absolutely respect the 12 nautical miles sea territory of China in all matters with the People's Republic of China in the East Sea.
Sincerely,
Hanoi 14-9-1958.
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Translation of Vietnamese government's diplomatic document (shown above) into Mandarin/中文:
越南民主共和国承中华人民共和国在1958年9月4日关于中国领海主张的各项条款。越南民主共和国尊重,并且将要求所有越南部门尊重中华人民共和国在东海(我南海)12海里的领海的领海。
敬礼
河内,1958年9月14日
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"Declaration of the Government of the People's Republic of China on China's Territorial Sea (September 4,1958)" in Chinese as follows:
文中所提到的中华人民共和国在1958年9月4日关于中国领海主张的各项条款如下:
中华人民共和国政府宣布
  (一)中华人民共和国的领海宽度为12海里。这项规定适用于中华人民共和国的一切领土,包括中国大陆及 其沿海岛屿,和同大陆及其沿海岛屿隔有公海的台湾及其周围各岛、澎湖列岛、东沙群岛、西沙群岛、中沙群岛、 南沙群岛以及其他属于中国的岛屿。
  (二)中国大陆及其沿海岛屿的领海以连接大陆岸上和沿海岸外缘岛屿上各基点之间的各直线为基线,从基线 向外延伸12海里的水域是中国的领海。在基线以内的水域,包括渤海湾、琼州海峡在内、都是中国的内海、在基 线以内的岛屿,包括东引岛、高登岛、马祖列岛、白犬列岛、乌岳岛、大小金门岛、大担岛、二担岛、东碇岛在内 ,都是中国的内海。
  (三)一切外国飞机和军用船舶,未经中华人民共和国政府的许可,不得进入中国的领海和领海 上空。
  任何外国船舶在中国领海航行,必须遵守中华人民共和国政府的有关法令。
  (四)以上(一)(二)两项规定的原则同样适用于台湾及其周围各岛、澎湖列岛、东沙群岛、西沙群岛、南 沙群岛以及其他属于中国的岛屿。
  台湾和澎湖地区现在仍然被美国武力侵占,这是侵犯中华人民共和国领土完整的和主权的非法行为。台湾和澎 湖等地尚待收复,中华人民共和国政府有权采取一切适当的方法在适当的时候,收复这些地区,这是中国的内政, 不容外国干涉。
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Translation:
Declaration of the Government of the People's Republic of China on China's Territorial Sea (September 4,1958)
  The Government of the People's Republic of China declares:
  1. The breadth of the territorial sea of the People's Republic of China shall be twelve nautical miles. This provision applies to all territories of the People's Republic of China including the Chinese mainland and its coastal islands,as well as Taiwan and its surrounding islands,the Penghu Islands,the Dongsha Islands,the Xisha Islands,the Zhongsha Islands,the Nansha Islands and all other islands belonging to China which are separated from the mainland and its coastal islands by the high seas.
  2. China's territorial sea along the mainland and its coastal islands takes as its baseline the line composed of the straight lines connecting bas-points on the mainland coast and on the outermost coastal islands; the water area extending twelve nautical miles outward from this baseline is China's territorial sea. The water areas inside the baseline,including Bohai Bayand the Chiung chow Straits,are Chinese inland waters. The islands inside the base line,including Tungyin Island,Kaoteng Island,the Matsu Islands,the Paichuan Islands,Wuchiu Island,the Grater And Lesser Quemoy Islands,Tatan Island,Erhtan Island and Tungting Island,are islands of the Chinese inland waters.
  3. No foreign aircraft and no foreign vessels for military use may enter China's territorial sea and the air space above it without the permission of the Government of the People's Republic of China.
  While navigation Chinese territorial sea,every foreign vessel must observe the relevant laws of the People's Republic of China and regulations of its government.
  4. The principles provided in paragraphs (2) and (3) apply also to Taiwan and its surrounding islands,the Penghu Islands,the Dongsha Islands,the Xisha Islands,the Zhongsha Islands,the Nansha islands, and all other islands belonging to China.
  The Taiwan and Penghu areas are still occupied by the United States armed force. This is anunlawful encroachment on the territorial integrity and sovereignty of the People's Republic of China. Taiwan,Penghu and such other areas are yet to be recovered,and the Government of the People's Republic of China has the right to recover these area by all suitable means at a suitable time. This is China's internal affair,in which no foreign interference is tolerated.
[Note: Thank you to FrankLau for the post and translation.]