Being bilingual is not enough
Someone may speak two languages well enough to communicate with native speakers, but the skills demanded of translators go beyond effective communication.
It's not only words
Translators must bridge linguistic and cultural gaps between two groups that cannot speak the same language. This means translating concepts, instead of providing a literal word-for-word version.
Here is a well-known example: Pepsi wanted to translate their latest slogan to increase sales in China. "Come Alive With The Pepsi Generation" somehow ended up being presented to Chinese consumers as "Pepsi Will Bring Your Ancestors Back From The Dead."
Other cases may not only sound scary, but fatal: instructions on a medicine bottle read "Adults: 1 tablet 3 times a day until passing away." The intention was to say that the dose should be taken "until symptoms pass..."
False friends are the root of the problem
A vast majority of Western vocabulary comes from Latin and Greek origin; therefore, many words may look similar in several languages, but have completely different meanings.