What would the official and unoffical name of Japan be in both Japanese, Tradtional Chinese, Mongolian, Manchurian and Persian along with its government style, quality of life, territories, defence and armed forces, national languages and dialects and industrial development in a world set of an realistic, plausible alternate history scenario for both Code Geass, Muv-Luv, High School DxD, and Zettai Karen Children and real life for a crossover fanfic of said franchises,  completely set in the world of the alternate history secenario with absolutely no references to real life, OTL or that this is even an alternate history timeline at all, when the Tang dynasty and the Tang dynasty's reigning imperial family, the Imperial House of Li, along with loyalists in the Tang army and the Tang government, aristocracy, nobility and royalty flees to the nation of Yamato in the Japanese Home Islands, Korea and Russia's Sahlakin and the Kuril Islands sometime after the Tang dynasty's collapse in 908 AD and subsequently takes it over in an attempt to reclaim the China region, with the Tang dynasty's Han Chinese, Mongolian, Hui Muslim and Manchu aristocracy, nobility and royalty intermarrying with the ethnic Korean and Japanese aristocracy, royalty and nobility in Korea, the ethnic Japanese aristocracy, nobility and royalty in the Japanese Home Islands and the Imperial House of Li intermarrying with Yamato's reigning imperial family, the Imperial House of Great Yamato/Minamoto dynasty as the decades and centuries go by, with the combination of Tang bureaucratic, military, industrial and economic achievements with indigenous Yamato government, military, industrial and technological achievements along with the mixing of the Tang dynasty's de-facto state religion of Sunni Islam along with Manchurian and Mongolian Shamanism, Confucianism and Japan's indigenous Shinto religion and Zen Buddhism and the slow almagation of the Manchu, Mongolian, Traditional Chinese and Persian languages with Japanese and Korean creating a strange hybrid state that is for all intents and purposes, the Tang dynasty but simply relocated to Yamato but slowly becoming less of a "Chinese dynasty" and more of a "Japanese nation state whose formation was given an additional push by the conquest of Yamato by the Tang, which itself was absorbed into Yamato as the centuries go by", slowly being recognized as a independent state from the Chinese dynasties and China in general from the Song dynasty onwards even as it keeps the Silk Road open and tries to maintain the Tang tributary system through Central Asia, although the Tang dynasty in exile is eventually absorbed into Yamato as the centuries go by, becoming this timeline's version of the modern state of Japan, which [this timeline's version of Japan] continues on as in real life, [this timeline's version of Japan] slowly forgetting its origins as a Yamato that was conquered by but eventually absorbed the Tang dynasty into itself even as it declares itself to be the true inheritor of Chinese civilization built by the Qin and later the Han dynasty and exemplified in the Tang dynasty, with the clan heads of the Imperial House of Great Yamato reigning over Japan as Japanese Emperors and Empresses respectively being the true successors to the Mandate of Heaven and emphasising Japan's status as the true successor to the Tang dynasty, which is viewed with ironic amusement by the aristocracy, royalty and reigning emperors and Empresses of the various dynasties ruling over China as the centuries go by, as they and the people of China view this timeline's version of Japan as the Tang dynasty that conquered but was eventually absorbed by Yamato, becoming Japan yet still being the Tang dynasty at the same time, almagating, improving and eventually subsuming Confucianism into Zen Buddhism and Manchu and Mongolian Shamanism, along with Sunni Islam and Chinese folk religion into Shinto but still keeping true to the nature of Imperial China at the same time, while also simultaneously maintaining a distinctively Japanese imperial and later shogunal government structure with the Japanese Emperors and Empresses having the title of Emperors and Empresses of Japan along with the combined positions of Emperors and Empresses of the Tang dynasty and thus being Emperors and Empresses of the eighteen provinces of Han China in addition which was essentially merged with the position of Khan due to the Imperial House of Li's Manchurian-Mongolian origins along with being the Sultans of the Tang dynasty, and the various shoguns ruling over Japan over the centuries having and acting as Caliphs of the Tang Empire in addition to being shoguns of Japan, independent from China and the China region in all but name but being the last remnants of a long gone Chinese dynasty, a walking contradiction,  although the Ming tributary kingdom of the Royal House of Jeongi Yi-dynasty ruled Joeson Kingdom is never formed by rouge Korean military commander Yi-Seong Gye in  July 1392 like in real life in this timeline and Korea remains a part of Japan from the rest of 1392 and beyond,  but Japan is the one to conquer the Tungning Kingdom in Southwestern Taiwan and the Pengu Islands instead of the Qing like in real life in this timeline with the Great Japanese Empire formed by the Meiji Restoration in 1868 being under the local governance of the Tokugawa shogunate like in canon Muv-Luv and unlike in real life due to the Meiji Restoration being a compromise between pro-Minamoto modernizers and progressives and pro-Tokugawa feudal traditional conservatives with the heads of the Tokugawa clan maintaining their positions as shoguns but the 5 Regent Houses of Koubuin, Ikaruga, Saionji, Kujo, and Takatsukasa are split off from and co-exist alongside the main branches of the Tokugawa clan like in canon Muv-Luv although the industrial, economic and military development after the Meiji Restoration that made Japan into a world power in real life still happens.

The Great Japanese Empire survives and simultaneously maintains control of Taiwan and the Pengu Islands, the Korean peninsula and Russia's Sahlakin and the Kuril Islands post-1947 into the 20th century and then into the 21th century and beyond along with maintaining control of the Beiyang Government of the first Republic of China as the Reorganized National Government of the Republic of China in Han China, the Empire of Manchukuo in Manchuria, Mengjiang in Inner Mongolia's Chahar and Suiyan regions, the Second Phillipine Republic in the Phillipines, the Empire of Vietnam in Vietnam, the Kingdoms of Laos and Cambodia in Laos and Cambodia, the Provisional Government of Free India in India Proper, the Yunnan Clique in Yunnan, the Kingdoms of Laos and Cambodia in Laos and Cambodia, and the Japanese East Indies in Indonesia  post-1947 into the 20th century and then into the 21th century and beyond, which [the Reorganized National Government of the Republic of China in Han China, the Empire of Manchukuo in Manchuria, Mengjiang in Inner Mongolia's Chahar and Suiyan regions, the Yunnan Clique in Yunnan, the Second Phillipine Republic in the Phillipines, the Kingdoms of Laos and Cambodia in Laos and Cambodia, the Provisional Government of Free India in India Proper and the Japanese East Indies in Indonesia] also all [the Reorganized National Government of the Republic of China in Han China, the Empire of Manchukuo in Manchuria, Mengjiang in Inner Mongolia's Chahar and Suiyan regions, the Second Phillipine Republic in the Phillipines, the Kingdoms of Laos and Cambodia in Laos and Cambodia, the Yunnan Clique in Yunnan the Provisional Government of Free India in India Proper and the Japanese East Indies in Indonesia] survive post-1947 into the 20th century and then into the 21th century and beyond due to the Reorganized National Government of the Republic of China's Peacekeeping Army pushing into the Kuomingtang-ruled Nationalist China's de-facto capital of Nanning and Nationalist China's wartime capital of Chongqing with Japanese help while also capturing Nationalist China's puppet state of Xibei San Ma's in Central China's capital of Xining, causing confusion in the Nationalist ranks and causing US and Soviet troops stationed in the China region to formally surrender to the Japanese just as the Japanese are preparing for conditional surrender to the Americans, with the Nationalist Chinese National Police Agency and Nationalist China's military intelligence agency acting as a foreign intelligence service and domestic secret police agency, the Bureau of Investigations and Statistics, both switching loyalty from the Kuomingtang and Nationalist China to the Reorganized National Government of the Republic of China ending the 1928-1947 Second Sino-Japanese war with a Japanese, Manchurian, Inner Mongolian, Yunnanese and Beiyang Government victory over the British Empire, the USA and the Soviet Union along with Nationalist China unlike in both real life and canon Muv-Luv

This causes confusion and chaos among the lower ranks of American and Soviet troops and intelligence officers fighting against the Japanese as the Soviets abandon the Mongolian People's Republic in Outer Mongolia and the Xingjiang Clique in East Turkestan while the Second Philippine Republic unites all of the Philippines under its rule.

When the then-Nationalist Chinese chairman, Lin Sen, and the then Nationalist Chinese head of state and leader of the Chinese armed forces' Chiang Kai Shek's second in command who had control over the National Chinese Armed Forces, Li Zongren, both announce the surrender of Nationalist China along with US, Soviet and British troops alongside the forces of the British aligned CCP to the Reorganized National Government of China and Japanese forces in the China region and Nationalist China recognizing Manchukuo and Mengjiang as legitimate states and the Reorganized National Government of the Republic of China as the legitimate government of Han China in front of te National Chinese unicameral legislature, the Legislative Yuan, in a public broadcast done via radio and television to the entire world, the truth is made clear to US soldiers distracted by easy victories in the Pacific and Soviet soldiers preparing for an invasion of Manchukuo. 

As the Bogd Khan-ruled Bogd Khanate of Mongolia is restored in Outer Mongolia and Inner Mongolia minus Chahar and Suiyan and the Ottoman Empire is restored in East Turkestan via popular revolutions after the fall of the Mongolian People's Republic and the Xingjiang Clique, and the Soviet Union collapses to a mishandled nuclear missile system sending an experimental German nuclear-tipped ICBM towards Moscow, three more towards London, New York and Washington D.C, allowing the European Axis to reclaim all of Eastern Europe from the Baltics and Poland to the borders of European Russia at the Urals along with the Caucuses from the Soviets, reclaim France, the Benelux nations, Luxembourg, Norway and even conquer Sweden and even seize Central Asia and even conquer the British Empire, with the British government fleeing to Canada in the process, American soldiers return home to the USA, Soviet soldiers flee the former Soviet Empire to the rump Soviet Republic in Siberia as Japan restores the Far Eastern Republic on the Russian East Coast while giving Outer Manchuria to Manchukuo and British soldiers follow the British government to Canada, hatred and vengeance against Japan on their minds and a "stab in the back" mentality against right wing evangelicals, Jews, and Arabs for causing the Allied loss in WW2 in this timeline, ironically similar to what caused the Nazi rise to power in Weimar Germany arising among them. 

As the Nazis make the Northern United States into a German protectorate in all but name, the transformation of the Canadian-based British Empire in exile into the Holy Brittanian Empire from Code Geass is complete with British exile-controlled Canada seizes the former Confederate Southern American States with the help of the Blue Dog Democrats and their paramilitary wing, the KKK.

Japan liberates the Kingdom of Hawaii in Hawaii and sometime later forms the federal parliamentary constitutional imperial monarchy of the Pacific States of America in the American West Coast. 

The restoration of the Hohenzollern-ruled German Empire as the government of Nazi Germany with the liberation of Austria from Germany and the subsequent restoration of the Austria-Hungarian Empire as the government of its pre and WW1-era former territories after the disapperance of Hitler sometime in 1963 stabilizes Europe and a three-way cold war between Axis-controlled Europe, Japanese-controlled East Asia and Brittanian-controlled North America begins.

N.B: The formal and informal name for Japan must fit Traditional and Imperial Japan era along with Traditional Chinese dynastic state naming principles and also take in mind the fact that Brittania was the name of the British Isles given to them by the Romans before, during and after Roman rule and conquest, which became associated with Great Britain and its colonial empire before becoming the formal name of the British Empire in exile from Code Geass, a similar principle applies to the Tang Empire in this timeline which fled to and conquered Yamato after the Tang dynasty's fall before becoming absorbed into Yamato itself, paving the way for the formation of this timeline's version of Japan. The informal name of Japan is what the rest of the world calls it just as how the Holy Brittanian Empire is officially called the Holy Brittanian Empire but legally is still the UK occupying Canada and Jamaica in preparation for return to the British Isles.