Grammar:
The language of Quran is Arabic.
It is not bounded by ordinary rules as it is neither verse nor prose.
https://nosurahlikeit.wordpress.com/2014/10/24/the-inimitable-literature-neither-versenor-poetry/
Foreign words:
If We had made it a foreign Quran, they would have said, ‘If only its verses were clear! What? Foreign speech to an Arab?’ Say, ‘It is guidance and healing for those who have faith, but the ears of the disbelievers are heavy, they are blind to it, it is as if they are being called from a distant place.’ (41:44)
Foreign Quran means the Quran in completely another language ,which is not the case.
[It is] an Arabic Qur'an, without any deviance that they might become righteous.(39:28)
Arabic Quran means the language is Arabic.It does not mean that any single foreign word would make the whole text foreign.
He lives in Madinah.
The language is English, though the word Madinah is actually arabic word.
This word does not make the whole sentence arabic.Same is for Quran.
The words with different origins also do not make the Quran in Foreign language,rather they reflect the normal language of the Arabic speaking community of that time.