You don't put an article in front of the profession when you're saying what occupation someone has, unless there's an adjective attached to it. So, "hon är en bra tolk" = "she is a good interpreter".
Technically I suppose you can use "en" in this sentence, but that gives a slightly different nuance to me. It's the difference between "This tall woman we're talking about is an interpreter" (without article) and "See that tall woman over there? She's an interpreter" (with article). That may just be my imagination though so take this whole paragraph with a grain of salt.
Ok, let's see if I can get this right... Lång is used for indefinites or when it's after the verb, e.g.: En lång kvinna. Kvinnan är lång. Same with långt, but for ett-words: Ett långt.... äpple (doesn't make much sense, I know :-D). Äpplet är långt. Långa is for definites and plural, e.g.: Den långa kvinnan. Långa kvinnor. Kvinnorna är långa. You can use långa for any grammatical gender, but for masculines you may use långe, e.g.: Den långe mannen. (I don't know if it's also correct to say Långe män and Männen är långe.)
Please correct me if I'm wrong :-)
We almost always use the separate article too when there's an adjective. The exception is some expressions that function like names. Example: Vita huset = 'The white house' – if you're talking about the house where the US President lives – if you're talking about some other white house, you need the article.