Anyone who is never at fault in what they say is perfect, able to keep their whole body in check.
James is a down to business, brass tack kind of book, that gets to the point without pulling punches. One know for sure when reading James that there’s no gray when it comes to how we should think and what we should do. That should all be one, otherwise we’re complicit with evil. In fact the reason we struggle with doubt (I would say from scripture and from life, one of the possible reasons) is because we’re not committed to God, but doubleminded in all our ways (James 1).
It does seem like from scripture, as in the passage quoted above, the tongue or what we say has a unique place. Of course, just as Jesus made clear, the tongue is the vehicle of the heart. So to address the problem of an unhelpful, evil tongue, one has to address the problem of the heart. That is a large subject in itself. What is key there is the gospel and the change that gospel brings through faith and baptism into Christ’s death and resurrection. So that by the Spirit we can put to death the misdeeds of the body, including the tongue.
It is indeed an impressive thought that the tongue is the gauge whereby we can see just how far we have to go, our spiritual growth. We won’t reach perfection in this life; indeed we stumble in many ways and we need ongoing forgiveness of sin. At the same time, we don’t have to let sin rule over us, which means no unruly tongue. Since we are under grace. While we work on this, a good rule of thumb is to remain in silence as our default, speaking only what is hopefully pleasing to the Lord and good and helpful to those who listen. And a good place to start is wherever we may have the most sin in this area. The Lord will help us, as we set our hearts and minds and lives in this direction.