First, it is indeed 100% essential for a missionary to speak the language. I, Matt, have been certified by the University of Central Florida as a Spanish-English interpreter, and I can tell you from my own personal experience that all interpreters fall short of getting the full message across. Professional interpreters often fail to correctly and fully convey the intended meaning of what the original language says—imagine someone without professional training and practice! They will miss part of the intended message, and “fill in the blanks” by guessing what the missionary might have said. More than once, I have seen a critical points in a sermon missed by an interpreter.
If a missionary is not comfortable with the Word of God getting muffled or confused, then that missionary has to learn the language of the people he’s trying to reach. That much is basically indisputable. But if God has put missions on your heart, should you only try to reach countries where you already speak the language? Not even close!! Adoniram Judson spent three long years learning Burmese; William Carey spent the first six years from his arrival in India learning Bengali and translating the New Testament. I doubt many would consider them to be ineffective missionaries!
Yes, if God is calling you to be a missionary, then you should be a missionary. And if you should be a missionary, then you should learn the language. But I, Matt, can say from personal experience that any language IS learnable. It IS doable. This is not something that only a select few can do. With a couple of years of hard work and dedication, anyone could learn a new language—and this isn’t even considering the fact that we have a Holy Ghost that we know will continue to perfect us and make us better for doing the work of God on this earth. If God has called you to missions, do not use the language as a cop-out! Be strong and courageous!