Language undergoes change at a rate easily visible in a lifetime. When I learned the word, it was spelled coöperation. Then they dropped umlauts ('dieresis', in English) more or less completely by the time I was through college. ('Reënter' is a similar outmoded form.)
Somewhere along the way, some people, unhappy with seeing two o's together at the beginning of the word, which might induce them to start off saying /KOOP/ (to rhyme with 'soup') in the first syllable, decided that we needed a hyphen there to break up a long 'o' followed by a short one.
Others thought that it was unnecessary (I among them) and that you tend to see the whole word and know how to pronounce it, warts and all (god knows English has all sorts of bizarre pronunciations that have nothing to do with the written form).
The word hasn't completely settled down, but I would predict that it will end up plain 'cooperation' with no dieresis and no hyphen, since English words move in the direction of less punctuation and accents, and I can't think of one case of a word moving in the opposite direction.
Until the battle is settled, I suggest you follow the requirements of the style guide governing the translation or company you are working with, and if it's just for your own usage, then pick which one you like, and stick with it, at least until the preponderance of native writers seem to agree on one or the other.