I am currently working on a voluntary translation project for a game. We're working in a team of ~5 people per language (3 active translators in our case) in a Smartcat project. However, one of the translators (also, the one that does most of the translating) tends to deliver very poor quality results, i.e. bad grammar, missing punctuation etc., so that I need to go over most of his translations again to fix these errors.
My question here is: what would be the best way to communicate he might not be the best fit for the job without it coming across as insulting? And should I rather start talking to him or the project coordinator?
I know it's a voluntary job without pay and we can't expect the best quality, however it is still the official translation for the game and it's a game I care deeply about and I want the quality to be as high as possible.
Edit (I don't have enough reputation to comment yet, so I'm going to have to answer this way) : thank you for the suggestion, but as I am not in any position of authority, the answers from Politely telling an incompetent software project volunteer they are too inexperienced don't really apply to my situation.