I’m not a professional.
Literally translated to German without any profession (teacher, hairdresser … whatever) given in the end that sounds like a lady insisting… ehem 
… that she is not a whore.
No similar problem in either English or Italian?
Well, certainly not in English. I HAVE heard the word used in English to denote what is also described as “the world’s oldest profession” (not coincidentally), but not for many, many years. I think it may have fallen out of favour as attitudes to that line of work have changed. (And I doubt that many younger people would even “get” the reference unless it was explained.)
More importantly, there is an overwhelming number of contexts in which it would NOT mean that in English, and would generally just be used by someone who is either (a) not getting paid or (b) does not have special training. Examples:
A: “Your photos are really good!” B: “Well, I’m not a professional.” (Meaning I don’t sell them and have never been trained in photography.)
A: “You did a really good job bandaging that wound!” B: “Thanks, but I’m not a professional”. (Meaning I’ve had first aid training, but I don’t work as a nurse or a doctor and therefore it’s not “professionally” bandaged.)
So, as you can imagine, the number of potential sentences where it means something OTHER than that vastly outweigh the number where it does.
Similarly I’ve not HEARD of the meaning being commonly used in Italian - there is another P word that I hear far more commonly for that – but I’ll leave commentary about that to someone who lives in the country.
I think the difference to German is the absence of a gender in English:
Let me explain:
In your bandaging situation, in German it would be totally okay to say:
“Danke, aber ich mache das nicht professionell.” (Thanks, but I am not doing that professionally.)
or
“Danke, aber bin kein Profi.” (There is no female form of the short “Profi”, so both a woman or a man can say that.)
But any woman would say
“Danke, aber ich keine professionelle Krankenschwester” instead of “Danke, aber ich bin keine Professionelle”. (Note the change from lower to upper case, but unfortunately you cannot hear that.)
What an excellent answer! 