I often hear people saying "due euro", even if "due euri" should be used.
Can anyone explain which one is correct, maybe adding a comment in reference to the differences in regional usage?
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Depending on the language euro is (or not) an invariant noun.
The European Union specifies that, in official documents, euro should be pluralized as euro.
Personally, I only rarely heard Italian people using euri (most often as a joke), and it just sounds wrong to say it.
The plural of euro is euro; I think that was established by the Accademia della Crusca.
Sometimes, you hear the plural being used, and often it is done in a semi-serious tone; in other cases, it is done from people who are used to use the plural for currency names (which what normally happens; compare un dollaro with dieci dollari), and does the same for euro.
I'd like to point out that neither Crusca nor EU can “decide” anything about a language. A language is shaped by its speakers, not by official authorities, no matter how important.
The Accademia della Crusca is a respected research body, whose opinion is very valuable, but just like physicists don't decide the laws of nature but study them, so modern linguists don't fix pronunciations or grammar rules but study language and its variations.
As for European Union, it may decide about the formal and stylistic characteristics of its documents and other official items (banknotes, say), but cannot – nor claims to – decide how people speak or write in everyday life. (And even when some autocrat or party insisted on imposing rules on language, they ultimate failed. History is full of commercial, literary, political made-up words and phrases that have taken a life of their own, independently of the intentions of the originators.)
This said, a majority of Italian speakers use euro as an invariant noun, while a minority advocate euri (and actually there is a precedent: euro as the noun of a wind has euri, in the generalised sense of “winds”, as its regular plural. For instance, «No; tu dal giovin animo / il timor freddo escludi; / gli Euri sonanti il portino / nelle letee paludi», Ludovico Savioli).
I second @kiamlaluno's answer. The Accademia della Crusca explains that the correct plural form of euro is euro (claiming that it was actually the EU decision from 1998 to make exceptions for the plural form of euro in English, German, and Italian).