Why the khutbah is in Arabic
Arabic is not an arbitrary choice. It is the language in which the Qur'an was revealed, and the language in which the Prophet ﷺ delivered the first khutbah in Madinah.
إِنَّا أَنزَلْنَاهُ قُرْآنًا عَرَبِيًّا لَّعَلَّكُمْ تَعْقِلُونَ
"Indeed, We sent it down as an Arabic Qur'an, so that you may understand." - Qur'an 12:2 (Yūsuf)
The position of the madhāhib
The majority of classical scholars - Mālikī, Shāfi'ī and Ḥanbalī - hold that the core parts of the khutbah (the ḥamd, the ṣalāh upon the Prophet ﷺ, a verse of the Qur'an, and the exhortation to taqwā) must be delivered in Arabic when this is possible. The Ḥanafī school gives wider room for non-Arabic speech, but Arabic remains the preferred and normal practice there too.
The reason is not only legal. The khutbah ties every congregation on Friday to the umma worldwide - the same verses, the same phrasing, the same language the Prophet ﷺ himself chose.
وَمَا أَرْسَلْنَا مِن رَّسُولٍ إِلَّا بِلِسَانِ قَوْمِهِ لِيُبَيِّنَ لَهُمْ
"And We did not send any messenger except in the language of his people, so that he might make things clear to them." - Qur'an 14:4 (Ibrāhīm)
Where translation belongs
Keeping the khutbah in Arabic does not mean understanding is unimportant - the verse above says li-yubayyina lahum, "so that he might make things clear to them." Many contemporary scholars therefore permit and encourage a translation or explanation alongside the Arabic khutbah, especially in mosques with many converts or young Muslims who do not yet know Arabic.
This is exactly where Minbar sits: the khutbah stays in Arabic, as the Sunnah preserves it, and the congregation reads the meaning along on their own phones without the imam having to break the flow of the sermon.