Aboo Uthmaan has kindly transcribed a lecture given by Shaykh ‘Abdur-Rasheed Azhar, about the Arabic language and the importance of learning and teaching it:

So the term “Arabic” ended up referring to a language and not an ethnicity. Whoever learned and spoke Arabic, no matter from where he came or from which ethnicity he belonged to he was an Arab. If one did not learn or speak Arabic then he is a non-Arab even if his forefathers were from the pure Arabs. So being an Arab is not an inherited trait but rather an earned trait. Hence, it is a reference to the language and not to an ethnicity or lineage.