Conspiracy is already a crime. Your cucumber example wouldn't attract a response because nobody actually cares enough to prosecute it. For example if your cucumber was made of solid goal then you would become a target of preventive action.
Attempting to commit a crime is also a crime in many jurisdictions, and only requires one person - maybe a prosecutor could convince a court that Derbasti was going to steal something, as demonstrated in a HN comment, but the plan was foiled.
Also, perhaps it is a threat - if other comments could be used to show that he/she was threatening the local supermarket unless they do something, perhaps charges for threatening could be filed.
Edit: Your claim that this is limited to mainly English speaking countries is not even true.
"Most nations prosecute inchoate acts (such as conspiracy and incitation) in their domestic criminal law."
https://www.law.upenn.edu/journals/jil/jilp/articles/1-1_Ste...