What is a good language to know/learn when looking to become a translator?

This is not easy to answer. In general translations into languages that are spoken in countries with strong economies are in higher demand but rare languages could fetch higher rates if you are a free-lance translator. Since I assume you reside in the US, Spanish is a good language to study as there is high demand for Spanish translators (somewhat lower rates). German is also in relatively high demand and fetches good rates.
However, in order to become a good or excellent translator you should have a passion for that language and you will need to spend some time abroad in the country where that language is spoken. So in that respect I recommend to study the language you like most or where you like the culture and want to visit/stay some time to get really immersed. It will take you years to speak the language well enough in order to become a translator.

Hi Shawn,

Great question! I agree with Ms. Grothe's advice to consider both the economic opportunities in countries that speak the languages you are considering as well as the rarity of the language. Both Germany and Japan have strong economies with tons of interactions with the U.S. economy; on the other hand, a rarer language is in lower demand, but can offer higher compensation precisely because it is rare and there are fewer translators available. I do disagree about learning Spanish, however. Unless you are a native Spanish speaker already, then you will not truly be able to compete with bi-lingual Spanish/English speakers already working as translators in the U.S. Another thing you might consider is guidance from the U.S. Department of State on"critical need foreign languages." These are 13 languages that are considered very high need. Japanese is on this list, as well as Arabic and Chinese. Not German. :) Every year, the Department of State provides full scholarships for college students to study these language in the countries that they are spoken through the Critical Languages Scholarship program, http://www.clscholarship.org/. This can be a great opportunity to travel abroad, improve your language skills, and meet other students with similar passions and career interests. Best of luck!

Hi,
Great question and you have received some great answers above as well. As a former Military Recruiter and Army Sgt, I will suggest that learning any middle eastern language will be good for you as well as mention to you that the military can offer you the best language training in the world if you are interested in serving.
Good luck...

My biases are probably clear. Again I would agree that thinking about regional necessity is a good start, Spanish is obvious, but there will always be Spanish speakers. French is the second most TAUGHT language in the world. That means there are many people out there that speak their native tongue plus French, but not Spanish and perhaps not English. French is the official or co-official language of more than 70 countries world wide, many near and in the wider Middle East, Lebanon, Algeria and Tunisia for instance.

It is a working language of the U.N. N.A.T.O and the Olympics, Spanish is not. I once had a friend who traveled to South Korea, to teach English, tell me that she conversed more on the street in French than English, as she spoke little to no Korean.