Yeah, you're right. I did think EU membership was more aligned with use of the Euro (I only knew about the UK using the Pound but they're out). It's still true that each country chooses its currency but it would be just as weird to pick one country in the EU to represent Euros as picking one country to represent a language.
Same goes for Spanish. Which country's flag should represent it? Most people probably don't even know the flag of Spain and readers in Latin America might be offended.
I think the fact is that languages simply don't have visual symbols for them. They are spoken or written, not "looked at". Languages are linear, not 2-dimensional. Euros and Dollars have their symbol because that's what is printed on the 2D surface of bills.
