The Quran Verse
Surah 14:4
And We did not send any messenger except in the language of his people to state clearly for them...
This verse establishes a rule:
messengers are sent to their own people in their own language for clarity.
If Muhammad was sent for all of humanity—as later Surahs claim—then according to the logic of 14:4, he should have been given a revelation in all languages, or messengers should have been sent to every distinct linguistic group.
By being revealed only in "Clear Arabic," the Quran, by its own definition in 14:4, identifies itself as a local revelation specifically for the Arabs of the 7th century, not a universal message for a global audience.