Because this is not a negative review, you can add some length to it. But because it's not unremittingly positive, it does not have to be lengthy. The ideal length is probably about 50 to 100 words. If you want to say more, contact the writer in private. For self-published works, editing and republishing are usually pretty easy. Hence if you find a glaring translation error, the writer can fix it. You can save the day with your review.