IF anybody places a significant amount of weight or emphasis on their faith, yet everybody else can observe that person's language is foul and does not line up with how their faith says you should be speaking THEN that person has believed a lie and his/her testimony is worthless.
There are many ways we stumble in life. Said another way, the perfect man is able to keep the rest of his mind, body, and spirit in harmony IF he never says a hurtful, incorrect, or sinful thing. The tongue might be one of the smallest instruments on the body, taken as a whole, but it has the power to guide us into all kinds of trouble from the smallest of comments (spark) to the the most grandiose of speeches (boasts). Ideas and things might start in our minds one way but by the time they get to our tongue for speech they come out another.
With all that humanity has tamed, like the animal kingdom, one might think surely we could tame our own devices such as the tongue but alone no man can do this. Scripture paints a vicious picture of the power and usefulness of the tongue when it says - "The tongue also is a fire, a world of evil among the parts of the body. It corrupts the whole person, sets the whole course of his life on fire, and is itself set on fire by hell." (James 3:6) Is there any doubt? I hear Christians of my own church praising our Lord and Father and not so long after that taking His name in vain/disrespectfully and or cursing men instead of praying for them. James would ask - What kind of water-well are we? Are we a fresh water well OR are we a salt water well? Pick one or the other. Stand for nothing and you'll fall for anything. Stand for the wrong thing and you'll be damned. So, get it right - get your language right before God and men - "do not swear--not by heaven or by earth or by anything else. Let your "Yes" be yes, and your "No," no, or you will be condemned." (James 5:12)
IF we can guard our mind - where we think about things and accept things from outside stimuli then we are one step in the right direction. IF we can guard our heart - where we choose to believe what we read, hear and say then we are two steps ahead in the game. IF we guard our tongue, because our speech is so naturally bad that Scripture again says - "It is a restless evil, full of deadly poison." (James 3:8), then "Whoever believes in me, as the Scripture has said, streams of living water will flow from within him." (John 7:38). It is a partnership between the Believer and the Holy Spirit concerning the development of Holy and Righteous speech. I encourage Christians to show you are a Christian By Your Words.
Paraphrase of James 1:26 and 3:2-12