Historically the South China Sea was named that in English because the British just sort of considered everything along the coast "China." This included Thailand (then Siam) and Taiwan (then Formosa). After Vietnam fell into separate kingdoms in 1533 France and Spain claimed ownership of a huge chunk of the waters for quite a long time until the Japanese imperialist era started in 1868. Chinese assertion of ownership came about after negotiations in 1953 when France said Vietnam had no claim on anything offshore and wouldn't let the Japanese return the Spratly Islands to Vietnam (then French Indochina) and insisted Japan hand the islands over to the French directly.
The language used to describe and name things is vastly more powerful than people give it credit for. Just because the British were lazy in their administration and named it after something else nearby doesn't mean that the entire sea belongs to the current Chinese state.
The language used to describe and name things is vastly more powerful than people give it credit for. Just because the British were lazy in their administration and named it after something else nearby doesn't mean that the entire sea belongs to the current Chinese state.