At the moment I work as Portuguese Interpreter for an agency and I hate it. They take very long to pay and I could earn double the money if I would work independently.
Self-employed means you must be good at marketing your services, and have prospective clients you can call on. If you have been working through an agency, such contracts typically have non-compete clauses that you cannot contact or work for clients you served through the agency for an extended period (typically a year, could be 2 yrs.).
How much money do you have to invest in starting up a business? Can you afford to spend half your time trying to get clients? How much demand is there in your area for Portuguese interpreters and/or translators?