I have been told I have pleural effusion, caused by a lung infection, but I am on holiday can it wait til I get home to get treated?

It would be unwise. to wait as potentially there could be a rapid progression of both fluid production and spread of infection. It could be so rapid that you would die before appropriate management could even be initiated. You might be sad about missing holiday this year but at least you will be alive to have a holiday next year.
Depends . on your condition, if you are sick, requiring oxygen and hospital treatment, then you have to be treated, shouldn't wait, if otherwise, and the effusion is small and discovered on x-ray, and you're clinically fine and walking around, can be treated as an outpatient, then go ahead, effusion will resolve, happens with pneumonia, and other lung problems, that you want to evaluate when you return.