Hey Edmond,
Vic already answered with most of what I was going to say - a big thing to remember is the issue of duplicate content if you are making a direct translation. You probably want to keep all content under the same domain for potential future link-building efforts. Using the /en approach Vic suggested will help with this.
Bear in mind, however, that such an approach can result in duplicate content penalties (see: Panda) if you are not careful with the translation process. It might be better to paraphrase your content when translating it in order to avoid these penalties.
The rest depends on what aspects of the site you want to translate, where your markets are and what language your potential customers are likely to speak.
Feel free to touch base with any questions,
Rob