All perfect praise be to Allah, The Lord of the Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.
For the validity of Friday's prayer, it is conditioned that the Imam delivers two sermons between which he stops and sits briefly, as is the position of the Shafiee', Malkiee' and Hanbaliee' schools of thought and the majority of Muslim scholars. This is because Bukhari and Muslim reported that:" The Prophet (PBUH) used to deliver two Khutbas and sit in between them. Moreover, based on the fact that performing Friday prayer as one Raka` isn't valid and doesn`t free a person from liability so is delivering one sermon during Friday's prayer. In addition, if the Imam hasn't delivered the second sermon, whether intentionally or forgetfully, then people are obliged to remind him as if he forgot to perform a Raka' in an obligatory prayer, since their prayer won't be valid if they didn't do so, and they have to redo Friday's prayer as an afternoon prayer. Further, it is permissible for one of the praying persons, who has been listening to the first sermon from the very beginning, to deliver the second one on behalf of the Imam if the period between the two sermons wasn't too long and in this situation, Friday's prayer is considered valid for all praying persons.
Furthermore, if none delivered the second sermon on behalf of the Imam and the period between the two sermons was too long, then they have to assemble and to redo the whole Friday's prayer including the sermon, and if they didn't, then they are obliged to perform it as an afternoon prayer according to the majority of Muslim scholars.
According to Hanafie' school of thought, the second sermon of Friday's prayer is considered an act of sunnah and leaving it won't invalidate the prayer. However, to be on the safe side, people should abide by the first opinion and whoever adopt the second opinion won't held accountable.
In conclusion, the Imam should take into consideration the doctrine followed in any country in which he delivers his sermons in order to avoid dispute regarding religious matters; especially, for the common Muslims who are not aware of the accurate Juristic issues. And Allah knows best.