Translation:We host a competition.
"A competiton is hosted" should be accepted and is the best way in English of leaving out the doer of the action.
But, that would make the sentence passive. The word "competition" is specified as object by the 를 in "대회를" so it should translated as such, even if the ambiguous subject seems funky in English.
English passives are also passive, dang it, despite what the colleges may say. The best way is to say, "Somebody (on my side) hosted a tournament." I mean, we all know it's somebody. But in these languages it's generally first person in statements unless it's obvious it's not, say by means of formality or respect, so the doer is not really left out of the meaning, only the number.