We may find one on Browse Fonts - Google Fonts or http://www.fontspace.com/ - but first we must get some quotes with as many as possible unique characters to check.
I am currently checking with phrase “zażółć gęślą jaźń 강남스타일 日本ひらがな” (polish unique chars, korean hangul, japan kanji and hiragana).
And atm I found one (o_O) unicode font which supports Korean symbols.
…or we may have 2-3 fonts for each ‘task’. First for latin-extended and cyrillic, second for Japanese, third for Korean…