Is it wrong to guarantee that what we say is true?
Swearing an oath is forbidden
Matthew 5:34 But I say unto you, Swear not at all; neither by heaven; for it is God’s throne: 35 Nor by the earth; for it is his footstool: neither by Jerusalem; for it is the city of the great King. 36 Neither shalt thou swear by thy head, because thou canst not make one hair white or black. 37 But let your communication be, Yea, yea; Nay, nay: for whatsoever is more than these cometh of evil.
James 5:12 But above all things, my brethren, swear not, neither by heaven, neither by the earth, neither by any other oath: but let your yea be yea; and your nay, nay; lest ye fall into condemnation.
The use of bad language is forbidden
Matthew 5:44 But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
Romans 12:14 Bless them which persecute you: bless, and curse not.
Colossians 3:8 But now ye also put off all these; anger, wrath, malice, blasphemy, filthy communication out of your mouth.
Colossians 4:6 Let your speech be alway with grace, seasoned with salt, that ye may know how ye ought to answer every man.
Ephesians 4:29 Let no corrupt communication proceed out of your mouth, but that which is good to the use of edifying, that it may minister grace unto the hearers.
Ephesians 4:31 Let all bitterness, and wrath, and anger, and clamour, and evil speaking, be put away from you, with all malice:
Ephesians 5:4 Neither filthiness, nor foolish talking, nor jesting, which are not convenient: but rather giving of thanks.
1 Peter 2:1 Wherefore laying aside all malice, and all guile, and hypocrisies, and envies, and all evil speakings,
1 Peter 3:9 Not rendering evil for evil, or railing for railing: but contrariwise blessing; knowing that ye are thereunto called, that ye should inherit a blessing.
I hope you find this helpful.