It's a crappy crappy (old) joke.
Made unfunny by the fact that;
A. Are we joining the Euro? (not to my knowledge but the joke is so old that for sure we weren't when I first heard it).
B. I've never heard the phrase 'spending a penny' unless I was spending a penny, which obviously anyone with more than one brain hemisphere would not say 'penny' when the currency does not have that unit of currency.
C. Why on earth would it be called Euronating? That makes no sense whatsoever, it has nothing to do with the phrase 'spending a penny' or anything else.
D. Some pleb just thought the 'word' Euronating sounded very similar to urinating and made a horrible, crappy joke to try and explain it, this person was probably 12 years old.
http://www.phrases.org.uk/meanings/spend-a-penny.html
Your last three points are wrong

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