Assalaamu alaykum wa rahmatullaahi wa barakaatuh. If someone knows Arabic as well as another language, is it impermissible for him to converse in that other language except for a need or legitimate reason? Can he not talk in English or Urdu while the person with whom he is talking also knows Arabic? May Allah reward you. Assalaamu alaykum wa rahmatullaahi wa barakaatuh; fee amaanillaah.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

The Muslim should not speak in a language other than Arabic without need if he is able to speak Arabic, because the Arabic language is the slogan of Islam and its people.

Imaam Ash-Shaafi'i said, "…and if he speaks Arabic, then he should not call something by a non-Arabic name, because Allah has chosen the Arabic language and has revealed His Book in Arabic and made it the language of the seal of His Prophets (Muhammad) ; so for this reason we say: Everyone who can learn the Arabic language should learn it, because it is the first language which should be desired, without prohibiting anyone from speaking in a language other than Arabic."

Shaykh Ibn Taymiyyah said:

"As for the habitual discourse in a language other than Arabic, which is the slogan of Islam and the language of the Quran, until it becomes a habit for the country and its people, and the family in the house, and a man with his friend, and the people in the market, or the rulers, or government members, or the people of Fiqh, then there is no doubt that this is disliked, as this constitutes imitation of the non-Arabs, which is disliked, as we have already mentioned … know that being accustomed to a given language greatly affects the mind, the morals, and the religion, and it also affects imitating (i.e. weakens trying to be similar to) the first of this nation, who were the Companions and the Taabi’is (generation following that of the Companions), while being in conformity with them enhances the mind (reason), religion, and morals."

Allah knows best.