There is an obvious importance for people to know the language of the locations that they are providing missions to. The more you can learn the local language, the better off you will be. Spanish is one of the more prevalent languages out there in terms of popular languages for missionaries. If you want to learn Spanish then you can give one of these programs a try: Rocket Spanish, Tell Me More, and the ever-popular Rosetta Stone.
The importance of knowing the local language is obvious; the more you can connect with the local people, the better off they will be in receiving the message that you would like them to hear.
An interesting quote that I recently read goes thus:
But we also need a new theology of the Logos. God has never left humankind alone in its pilgrimage. All human beings are created in the image of God (Gen. 1:27), and they experience this self-manifestation of God. They try to live in that image of God within their own cultural surroundings, and they express this in various “ways,” the religions of the world. God’s Spirit and Word, the Logos, is active among us from the very beginning of creation until the end of time. The Logos is present in all of us (John 1:4), is eminently present in Jesus, and will be with us until the Second Coming. Followers of the Way of Jesus are not living in a ghetto, separated from other human beings. They are copilgrims with a special task and mission: to be faithful in the search for fullness, helping themselves and others to grow in God’s way. Christians should be a light, leaven, a mustard seed.
~Camps, Arnulf. “My pilgrimage in mission.”
Therefore, learning the language of the locals is so important even if to see things from their perspective. In learning about linguistic anthropology we know that sometimes languages can influence ways of thought. One example is the gender system of many languages. Spanish speakers will tend to think of inanimate objects as having either masculine or feminine qualities depending on the gender that they are assigned in Spanish.
Ultimately this means that we have a long way to go in terms of fully committing to service in any language. You must pour yourself into it as into earthen vessels.
Obviously translation has a huge impact, especially in terms of biblical translations. Translations of the bible, even to one single word, can alter the fates of many. Therefore we must take care in how we translate our ideas, and keep one ear towards the Spirit for guidance.