Lying or using bad language does not invalidate the fast. Such words and actions are not among the acts that break the fast.
However, while fasting, a person must strive to control all of their limbs and senses, even their thoughts and imagination, avoiding sinful and useless (mālâyānī) things. The Messenger of Allah (peace and blessings be upon him) stated that true fasting is only achieved in this way:
“Whoever does not give up lying and acting upon falsehood while fasting, Allah has no need for him to abstain from food and drink.”
(al-Bukhārī, “Ṣawm,” 8)