The Quran

وَلَو جَعَلناهُ قُرآنًا أَعجَمِيًّا لَقالوا لَولا فُصِّلَت آياتُهُ ۖ أَأَعجَمِيٌّ وَعَرَبِيٌّ ۗ قُل هُوَ لِلَّذينَ آمَنوا هُدًى وَشِفاءٌ ۖ وَالَّذينَ لا يُؤمِنونَ في آذانِهِم وَقرٌ وَهُوَ عَلَيهِم عَمًى ۚ أُولٰئِكَ يُنادَونَ مِن مَكانٍ بَعيدٍ

Muhammad Asad

Now if We had willed this [divine writ] to be a discourse in a non-Arabic tongue, they [who now reject it] would surely have said, "Why is it that its messages have not been spelled out clearly?¹ Why -[a message in] a non-Arabic tongue, and [its bearer] an Arab?" Say: "Unto all who have attained to faith, this [divine writ] is a guidance and a source of health; butas for those who will not believe in their ears is deafness, and so it remains obscure to them: they are [like people who are] being called from too far away.²

Sc., "in a tongue which we can understand". Since the Prophet was an Arab and lived in an Arabian environment, his message had to be expressed in the Arabic language, which the people to whom it was addressed in the first instance could understand: see in this connection note 72 on the first sentence of 13:37, as well as the first half of 14:4 - "never have We sent forth any apostle otherwise than [with a message] in his own people's tongue, so that he might make [the truth] clear unto them". Had the message of the Qur'an been formulated in a language other than Arabic, the opponents of the Prophet would have been justified in saying, "between us and thee is a barrier" (verse 5 of this surah).
Lit., "from a far-off place": i.e., they only hear the sound of the words, but cannot understand their meaning.