One says 10 Dollars or 10 Euros, but 10 Yen or 10 Yuan.
As noted in multiple comments, the premise of your question is faulty. Officially, the plural of Euro is Euro:
In Community legislative acts the plural forms of euro and cent are spelled without the s, notwithstanding normal English usage.
The official currency of The Peoples Republic of China is the Renminbi. Yuan is used as a synonym for Renminbi internationally and referred to by the abbreviation CNY. In China they use Yuan to describe currency in general somewhat like the British could refer to pounds as Sterling. This is a hold over from when Chinese currency was not Fiat and a Yuan was a silver coin that could be broken down into smaller amounts by a decimal system where they would say for instance .5 Yaun leaving no need changing Yuan to Yuans. I own some modern Chinese silver panda coins that have a face value of 10 Yuan which if i were to sell for spot silver price while in China I would receive Some amount of Renminbis. The Chinese currently refer to yen as Japanese Yuan, The Dollar the American Yuan, the Euro the European Yuan and so on. Hopefully what I said helps. I'm not sure about the yen part of the question sorry.
This is because they are invariant nouns. Examples of invariant currencies in plural form:
- Yen (Japan)
- Sen (fraction of a yen, Japan)
- Yuan (China)
- Jiao (fraction of a yuan, China)
- Fen (fraction of a jiao, China)
- Baht (Thailand)
- Satang (fraction of a baht, Thailand)
- Rand (South Africa)
- Quid (British slang)
And, here are some irregular plurals of some other currencies:
- Penny / pence (fraction of a pound, United Kindom only)
- Paisa / paise (fraction of a rupee in India and Pakistan)
- Drachma / drachmae or drachmas (Greece, formerly)
- Krona / kronor (Sweden)
- Krone / kroner (Norway and Denmark)
- Markka / Markkaa (Finland, formerly)
- Lira / lire (Italy, formerly)
- Real / reais (Brazil)