Is it permissible for someone who graduated from university in Arabic language by cheating to work as an Arabic language teacher, and is his conscience absolved by preparing himself for teaching after graduation?

Obtaining a certificate through cheating is forbidden (haram (definition)), and it is not permissible to work with it, especially in a teaching profession which requires competence and trustworthiness. This is because any deficiency in it will cause great, far-reaching, and widespread harm to students. Therefore, the questioner must repent to Allah and re-study so that they can succeed through their own effort and fulfill the trust in their profession.