The Holy Quran was revealed by God Almighty in the Classical Arabic language to Prophet Muhammad (peace be upon him).
Arabic is a Semitic language, and is spoken by around 250 million people worldwide. There are many dialects of Arabic, like Modern Standard Arabic, Egyptian Arabic, Gulf Arabic, Maghrebi Arabic, Levantine Arabic, and various others.
The people alive during the time of Prophet Muhammad (peace be upon him) spoke the native language of Arabic, and thus, the Holy Quran was revealed in Classical Arabic, which made it easy for the people to comprehend and contemplate on it.
Concerning this matter, God Almighty mentions in the Holy Quran:
Holy Quran; Surah 41; Verse 3:
3. كِتَابٌ فُصِّلَتْ آيَاتُهُ قُرْآنًا عَرَبِيًّا لِّقَوْمٍ يَعْلَمُونَ
A Book whereof the verses are explained in detail a Qur’an in Arabic for people who understand
Holy Quran; Surah 12; Verse 2:
2. إِنَّا أَنزَلْنَاهُ قُرْآنًا عَرَبِيًّا لَّعَلَّكُمْ تَعْقِلُونَ
We have sent it down as an Arabic Qur’an in order that ye may learn wisdom.
Holy Quran; Surah 39; Verse 28:
28. قُرْآنًا عَرَبِيًّا غَيْرَ ذِي عِوَجٍ لَّعَلَّهُمْ يَتَّقُونَ
(It is) a Qur’an in Arabic without any crookedness (therein): in order that they may guard against Evil.
Holy Quran; Surah 44; Verse 58:
58. فَإِنَّمَا يَسَّرْنَاهُ بِلِسَانِكَ لَعَلَّهُمْ يَتَذَكَّرُونَ
Verily We have made this (Qur’an) easy in thy tongue in order that they may give heed.
Even though the Holy Quran is found in the Arabic language, it remains the only book in the world that’s been memorized completely by millions of people – both by Arabs and non-Arabs alike.
Holy Quran; Surah 54; Verse 40:
40. وَلَقَدْ يَسَّرْنَا الْقُرْآنَ لِلذِّكْرِ فَهَلْ مِن مُّدَّكِرٍ
And We have indeed made the Qur’an easy to understand and remember: then is there any that will receive admonition?