It is true that only the Arkan of the khutbah must be in Arabic. However, Shaykh Sulayman al-Bujayrmi writes in his Hashiyah of Shaykh al-Islam's Sharh al-Manhaj, when the author says " what is meant are the integrals of the Khutbah" that this give the impression that if another language were to be used between the integrals it would not harm (the validity). Thereafter, al-Bujayrmi relates from Imam al-Ramli that this view (namely that foreign speech between the integrals does not harm) is warranted if the separation between the non-arabic speech and the arabic integrals is not a "long" separation, otherwise (i.e. if the non-arabic speech caused a long separation) it would be harmful due to the fact that it would interupt the continuity (between the integrals)
In fact, I recall one of my teachers in Tarim saying that any speech that is not in the interest of the Khutbah will invalidate it, such as ranting about politics etc, provided it prolongs the separation between integrals.
Allah Ta 'Ala knows best.
There is a fatwa from Indonesian Shafi'i Aleem, Habib Munzir ibn Fuad al Muwada.
If you are living in a place among Muslims from other madhaab, you should follow fiqh of madhaab that followed by those people. In example, if you live among Hanafis, you should salah in Jama'ah with them according to Hanafi fiqh.
Also, there are some similar fatawa that issued by other Shafi'i Ulama.
So, although Muslims people around you are not Shafi'is, you can offer salat-ul Juma'ah with them.