The Friday sermon is not merely a speech or a talk but is in fact an essential and integral part of the Friday prayer and such must be delivered as prescribed by the Prophet, that is, in the Arabic language. A similar example is that of Adhan (call to prayer) which must be called out in Arabic language. We know that the companions of the Messenger of Allah, peace and blessings of Allah be upon them all did not change the language of the Friday sermon to the native language of the lands they conquered even though there was obvious need for the people to understand the sermon being delivered to facilitate their learning of the new religion. In other words, Arabic is the official language of Islam so far as the religious rites are concerned even if it may present a significant problem in people comprehending it.
Though, certain other schools of Islamic Jurisprudence allow it to be delivered in languages other than Arabic, it is more precautious and safe to do so in Arabic as has been the practice of more than 1300 years of Islamic history.