(Al-Kahf) 18 : 93
حَتَّىٰ إِذَا بَلَغَ بَيْنَ السَّدَّيْنِ وَجَدَ مِنْ دُونِهِمَا قَوْمًا لَا يَكَادُونَ يَفْقَهُونَ قَوْلًا
Until when he reached between two mountains, he found beside them a people whose words were hard to understand.
The language of that people was very difficult to understand for Dul-Qarnayn and other people.