Ave Maria, gratia plena, Dominus tecum. Benedicta tu in mulieribus, et benedictus fructus ventris tui, Jesus. Sancta Maria, Mater Dei, ora pro nobis peccatoribus nunc et in hora mortis nostrae. Amen.

I am writing this to clarify whether it is a sin or not to use foul language.

It depends, not on the words themselves in se, but on the intention, whether that is a good intention or a bad one.

It is a mortal sin to use such language in the presence of those who are so weak-spirited that they will suffer scandal, and especially in front of children. It will be a mortal sin to use foul language when there is danger of spiritual ruin to oneself or another.

To use immodest words ex vano solatio, vel joco, is a venial sin.

If the words are very lascivious, I think one should act conscious of the much higher likelihood that hearers will suffer scandal.

It is worth remembering that those who habitually use impure words are likely inadvertently to use them in front of children, who, as a result of hearing such words, will commit a thousand sins.

I have taken this mostly from St Alphonsus.

Ave Maria, gratia plena, Dominus tecum. Benedicta tu in mulieribus, et benedictus fructus ventris tui, Jesus. Sancta Maria, Mater Dei, ora pro nobis peccatoribus nunc et in hora mortis nostrae. Amen.