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Even in discussions about modesty, almost incredibly, one sees some people speak immodestly - i.e. with vulgar language, expletives and other illicit and sinful behavior. Why is this? Shouldn't we, as Traditional Catholics, know that such speech does not befit Christian lips, especially lips that have received the Lord in Holy Communion? Thoughts?

"ON THE VICE OF SPEAKING IMMODESTLY – St. Alphonsus
ON THE VICE OF SPEAKING IMMODESTLY.
“He touched his tongue, …. and the string of his tongue was loosed.” MARK vii. 33, 35.

IN this day’s gospel St. Mark relates tlie miracle which our Saviour wrought in healing the man that was dumb by barely touching his tongue. “He touched his tongue and the string of his tongue was loosed.” From the last words we may infer that the man was not entirely dumb, but that his tongue was not free, or that his articulation was not distinct.
Hence St. Mark tells us, that after the miracle he spoke right. Let us make the application to ourselves. The dumb man stood in need of a miracle to loose his tongue, and to take away the impediment under which he laboured. But how many are there on whom God would confer a great grace, if he bound their tongues, that they might cease to speak immodestly! This vice does great injury to others. Secondly, it does great injury to themselves. These shall be the two points of this sermon ...

9. Be attentive, then, my brethren, and guard your selves against speaking immodestly, more than you would against death. Listen to the advice of the Holy Ghost: ”Make a balance for thy words, and a just bridle for thy mouth; and take heed lest thou slip with thy tongue and thy fall be incurable unto death.” (Eccl. xxvhi. 29, 30.)
”Make a balance” you must weigh your words before you utter them and”a bridle for thy mouth” when immodest words come to the tongue, you must suppress them; otherwise, by uttering them, you shall inflict on your own soul, and on the souls of others, a mortal and incurable wound. God has given you the tongue, not to offend him, but to praise and bless him.
”But, ” says St. Paul, “fornication and all uncleanness, let it not so much as be named among you, as becometh saints.” (Ephes. v. 3.) Mark the words”all uncleanness. ” We must not only abstain from obscene language and from every word of double meaning spoken in jest, but also from every improper word unbecoming a saint that is, a Christian. It is necessary to remark, that words of double meaning sometimes do greater evil than open obscenity, because the art with which they are spoken makes a deeper impression on, the mind.
10. Reflect, says St. Augustine, that your mouths are the mouths of Christians, which Jesus Christ has so often entered in the holy communion. Hence, you ought to have a horror of uttering all unchaste words, which are a diabolical poison. ”See, brethren, if it be just that, from the mouths of Christians, which the body of Christ enters, an immodest song, like diabolical poison, should proceed.” (Serm. xv., de Temp.) St. Paul says, that the language of a Christian should be always seasoned with salt. ”Let your speech be always in grace, seasoned with salt. ”(Col. iv. 6.)

Our conversation should be seasoned with words calculated to excite others not to offend, but to love God. ”Happy the tongue,” says St. Bernard, ”that knows only how to speak of holy things!”

Happy the tongue that knows only how to speak of God! brethren, be careful not only to abstain from all obscene language, but to avoid, as you would a plague, those who speak immodestly. When you hear any one begin to utter obscene words, follow the advice of the Holy Ghost: ”Hedge in thy ears with thorns: hear not a wicked tongue.” (Eccl. xxviii. 28.)

“Hedge in thy ears with thorns” that is, reprove with zeal the man who speaks obscenely; at least turn away your face, and show that you hate such language. Let us not be ashamed to appear to be followers of Jesus Christ, unless we wish Jesus Christ to be ashamed to bring us with him into Paradise" http://www.traditionalcatholicpriest.com/2017/08/19/vice-speaking-immodestly-st-alphonsus/

Indeed, my biggest problem is with the GD term, or the use of Our Lord's name as a curse word, especially when somebody injects the F word between Jesus and Christ.

Rest are vulgar of course and low class, but not nearly approaching the severity of these two (above) and the other blasphemous ones.

Vulgarity is an interesting topic.  Many of our current dirtiest four-letter words were simply normal words in Anglo-Saxon.  But why is "copulate" an OK word to use but the F word is not?  After the Norman invasion, the Normans/French considered the Anglos to be barbaric, and their language, filled with mono-syllabic words, to be crude, sounding to their ear like grunts and groans.  Perhaps they were just efficient?  :) Why use 3 syllables to say something when you can say it in 1?

In any case, our Jєωιѕн overlords have deemed it OK to say GD phrase on radio or television but you can't say sh*t ... not without getting a fine.  So a crude way of saying poop is worse than blaspheming God?

Are they sinful?  Are they mortally so?  To be clear I'm asking about swearwords other than the Lord's Name in vain.

I once heard a priest say that swearing was a mortal sin but I wasn't given a clear source on why.

I'm not certain.  My general impression that it would not be, per se, but I imagine that could be exacerbated into grave sin by the context or the severity.  I did not make it to the moral theology classes at seminary, though, so I would ask a priest.  Father Kramer is floating around the forum, so perhaps he would be one to ask.