wp_insert_post hook for a wordpress plugin

The right hook to use before inserting the post is wp_insert_post_data

function tr_insert_post($data){
    $post ['post_title'] = "the title: ".$post ['post_title'];
    return $post;
}

add_action('wp_insert_post_data', 'tr_insert_post',1,2); 

But I think that a better way to achieve that would be to save the post as is and translate later using a filter like the_content this way you save the post in the same language as you posted and you can edit it easily in that same language.