In some cases, you will not need to learn a new language. You may serve cross-culturally in the US, or in another English-speaking nation such as Great Britain. In other nations, English is a substantial second language, and you may be able to practically function with English in daily life. Even so, you will still not be communicating in people’s heart language, which is often a key to understanding the gospel.

In most places you will need to learn at least one, and in some places, two or three languages. For example, Moroccans predominantly speak Arabic, but Morocco was a French colony. French remains the language of medicine, business and law. You’ll need French if you anticipate visiting doctors or running a business. Then if you choose to work among Berbers, the original tribes of Morocco, you will also need to learn a regional Berber dialect.

It may be easy to view language learning as a hurdle to beginning ministry, when in fact it IS ministry. Missionaries should view language learning as working as unto the Lord. Your grasp of the local language will affect both your effectiveness and your satisfaction of living in a new country. Don’t succumb to the temptation of short cutting this vital part of ministry.

It depends on where you go and who you minister to, and sometimes it also depends on what your role is in that culture.

Most of the time, a missionary needs to learn another language. That is part of the "going" going involves moving and traveling and living among the people you will minister to, and so, yes, normally a person is going to learn another language, especially if the new culture/people group/country does not speak your language. In order to communicate the gospel and teach and disciple people in the Bible, first one has to learn their language and culture and be with them in order to communicate to them.

"make disciples of all nations" in Matthew 28:19 involves deeper language learning, and the word "nations" is the Greek word, ethna / εθνη - where we get "ethnic" and "ethnicity" from, and part of culture learning is language learning, a big part. Learning their language and culture and food and customs and history and poetry, is all applications of obeying the principle in 1 Corinthians 9:19-23 - "I have become all things to all men, in order that I may save some"; and "I have become their servant/slave", in order to share the gospel and disciple them. The commission is to disciple, not just share about Jesus and leave. If we are going to be obedient to Jesus' command, and it involves other nations and cultures, then it involves deep learning of their language and culture and so, yes, most of the time, one needs to learn another language.

But if you are a missionary to your own culture or another country that speaks your language, then you don't have to learn another language. And if your role is a role that does not require you to learn the language, for example, some people go full time to teach missionary children in English while the missionary couple or couples spend the time in language and culture learning. That role would not require you to learn the native language, but you probably will learn some things anyway, to live there. If one's role is in financial services for the mission or full time in building and administrative duties in another country, one may not have to learn the other language.