It's confusing because occupations take an adjectival form. "Je suis acteur." "Elle est actrice." If you wanted to use a definite pronoun, it would be "c'est des professeurs."
It's hard to know in this instance if the meaning is professional or professionals. I think both answers ought to be accepted.
With all due respect, the relevant issue, surely, is what English speakers would say in this situation. "We are professional, of course we can handle it!" and "We are professionals, of course we can handle it!" are both possible. That's why DL ought to accept both (without worrying about the otherwise interesting question of whether the French form is a noun or an adj.).
It should only be as close as possible if it's also grammatically correct as well as representative of how people actually speak. A technically "correct" and more-or-less verbatim translation that nobody would ever use doesn't make a lot of sense. And a correct speaker of American English would be very unlikely to say "professional" in this case, regardless of how French speakers treat adjectives or nouns with respect to careers. We would always use "professional" as an adjective to modify a noun, never to replace one: "We are professional athletes" or whatever, but never simply "We are professional." That would be an incomplete sentence. Professional what? We would always say "We are professionals" because the noun (professionals) would have to have plurality agreement with the pronoun (we). Likewise, "He is A professional." "He is professional" sounds pidgin. Even as an answer to a question, say "Are you guys amateur detectives?" "No, we are professional" doesn't sit quite right, and would probably not be said by anybody reasonably articulate. So, if "Je suis actrice" correctly translates to "I am AN actress" then "Nous sommes professionelles" correctly translates to "We are professionals."
(side rant: I really hate how DuoLingo yells to "stop the clutter" the minute the typing starts. Given how many people clearly post without reading the comments first, it really doesn't seem to serve any purpose other than scolding and probably only makes decent people feel bad and to discourage participation. Don't be so grumpy guys!)
I disagree. "We are professional" is certainly a used and useful complete sentence. I did answer "we are professionals" and missed the nuance of adjective versus noun, and agree that is the most common and natural statement with the noun. But the use of professional as a modifier is hardly pidgen, and only a small amount of context makes it sound right. You can be professional in your attire, behavior, demeanor, without being A professional.
"We are all sitting in the lobby awaiting our turn for the job interview. We are dressed sharply, are well groomed, have our resumes, and have good manners. We are professional." We are not professionals, because we don't have jobs.
You many not say "we are professional" in the dialect of English that you speak, however, that is not what this app is about. We are learning French and there are a lot of French ways of saying things that may seem odd to the native English speaking mind. So it doesn't matter if it doesn't feel natural to us in English, it is important that we understand the true meaning of the phrase rather than what is 'natural' in our certain dialect of English. :) Make sense?
Last two comments very well said. What we would or wouldn't say in English is irrelevant, if it is not inferred by the given French. Whilst it is true that professions act as adjectives, without need of a modifier, 'professional is not an occupation. We most certainly would hear 'we are professionals', where it is a noun, however to be a noun in French, we know that we need an article: 'Nous sommes des professionnels'. 'We are professional' is perfectly acceptable and well used in English, and is the correct translation of what is given here. Larousse confirms this, indicating that any use of 'professionnel' as a noun requires further qualification as to what one is a professional in; 'professionnel de la boxe' - professional boxer. The plural here is not acceptable as an English translation due to the lack of articles.
Your question is exactly what I was asking when I opened this discussion. As I read through all the comments, I pondered some ideas.
I'm thinking the distinction is that in "We are professionals" (which is what I just used as my answer and was informed I was incorrect) the word "professionals" is a noun that defines the pronoun "we." In the phrase "we are professional" the word "professional" is an adjective that describes or clarifies the pronoun "we."
Does that sort of make sense? (Guess I'm asking for validation or further clarification if I'm off target.)