There are problems with using many different sources for translations. One is that you may get different "voices" or interpretations between translators for the same text to be translated. If part of the text is translated by a 60-year old and part is translated by a teenager, they're most likely going to be quite different styles.
Unless you are an expert in the language (in which case you don't need someone else to translate in the first place), how do you know any particular user submitted translation is better than another?
In addition to Ergophobe's questions, I would ask:
3. Do you have any self-policing from your community to alert you to bad or incorrect translations?