As for becoming accustomed to talking to one another in a language other than Arabic, which is the symbol of Islam and the language of the Qur'an, so that this becomes a habit in the land, with one's family and household members, with one's friends, in the marketplace, when addressing government representatives or authority figures or when speaking to people of knowledge, undoubtedly this is makruh (disliked), because it involves being like the non-Arabs, which is makruh, as stated previously.
Hence when the early Muslims went to live in Syria and Egypt, where the people spoke Byzantine Greek, and in Iraq and Khurasan, where the people spoke Persian, and North Africa (al-Maghrib) where the people spoke Berber, they taught the people of those countries to speak Arabic, so that Arabic became the prevalent language in those lands, and all the people, Muslim and kuffar alike, spoke Arabic. Such was also the case in Khurasan in the past, then they became lax with regard to the language and got used to speaking Farsi until it became prevalent and Arabic was forgotten by most of them. Undoubtedly this is disliked.
The best way is to become accustomed to speaking Arabic so that the young people will learn it in their homes and schools, so that the symbol of Islam and its people will prevail. This will make it easier for the people of Islam to understand the Qur'an and Sunnah, and the words of the Salaf, unlike a person who gets used to speaking one language, then wants to learn another, and finds it difficult.
Know that being used to using a language has a clear and strong effect on one's thinking, behaviour and religious commitment. It also has an effect on making one resemble the early generations of this Ummah, the Companions and the Taabi'in. Being like them improves one's thinking, religious commitment and behaviour.
Moreover, the Arabic language itself is part of Islam, and knowing Arabic is an obligatory duty. If it is a duty to understand the Qur'an and Sunnah, and they cannot be understood without knowing Arabic, then the means that is needed to fulfil the duty is also obligatory.
There are things which are obligatory on all individuals (fard 'ayn), and others which are obligatory on the community or Ummah (fard kifayah, i.e. if some people fulfill them the rest are relieved of the obligation).
This is the meaning of the report narrated by Abu Bakr ibn Abi Shaybah who said: "Isa ibn Yunus told us from Thawr from 'Umar ibn Yazid that 'Umar wrote to Abu Musa al-Ash'ari radiallaahu 'anhu and said: 'Learn the Sunnah and learn Arabic; learn the Qur'an in Arabic for it is Arabic.' "
According to another hadith narrated from 'Umar radiallahu 'anhu, he said: "Learn Arabic for it is part of your Religion, and learn how the estate of the deceased should be divided (fara'id) for these are part of your Religion."
This command of 'Umar, to learn Arabic and the Shari'ah combines the things that are needed, for Religion involves understanding words and actions. Understanding Arabic is the way to understand the words of Islam, and understanding the Sunnah is the way to understand the actions of Islam ...
(s) Iqtida' as-Sirat al-Mustaqim (2/207)