Praise be to Allah.
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) was asked this question. He replied as follows.
Our view on learning English is that it is a means and it is a bad menans if the aim is bad. But the one thing that we must avoid is using it instead of Arabic. That is not permitted. We have heard some foolish people speaking English instead of Arabic, and some of the foolish people who are dazzled by the west, whom I regard as lackeys (of the west), teaching their children to use the greetings of the non-Muslims, and teaching them to say “bye-bye” when departing, and so on.
Because using this language instead of Arabic, which is the language of the Qur’aan and the noblest of languages, is haraam. It was narrated that the salaf forbade speaking in the tongues of the non-Arabs.
With regard to using it as a means of da’wah, there is no doubt that this is obligatory sometimes. I did not learn it and I wish that I had learned it. On some occasions I have found forced to use an interpreter who could not fully express what I wanted to say.
I will tell you a story of something that happened to me in the mosque of Jeddah airport, with some men from the Islamic Awareness organization. After Fajr prayer, we were speaking about the Tijaani sect and how it is a false sect that that apostatized from Islam. I started to say what I knew about them, then a man came and asked me for permission to interpret my words into Hausa. I gave him permission, and he started to interpret. Then a man came rushing in and said, “This man who is interpreting for you is praising the Tijaaniyyah.” I was astonished and I said, “Inna Lillaahi wa inna ilayhi raaji’oon (Truly, to Allaah we belong and truly, to Him we shall return).” If I had known this language I would not have needed these cheaters. The point is that knowing the language of the people whom you are addressing is undoubtedly important so that you can convey the information to them. Allaah says (interpretation of the meaning):
“And We sent not a Messenger except with the language of his people, in order that he might make (the Message) clear for them”