That said, I LOVE the idea of multiple currencies in theory. However most RPG games IMO don't even get one effing currency done right. Oftentimes gold (or whatever) becomes near-meaningless as you can often just buy absolutely everything you need as soon as you find a new store to buy stuff with. And if you can't, it's often only 1 piece per character at most (or just one piece period) that you cannot afford, and everything else is trivial, causing you to do the same thing (buy everything you need), and save up for one piece (what choice, what depth!) later on!
I want to make (or play lol) and rpg where stores have great equipment, the good equipment isn't just handed to you via quests or chests in dungeons, but you can't have it all. you have to pick and choose, wisely. Every gold you make is exciting because every gold you make leads to a meaningful and significant choice of upgrade of your choice.
This is usually caused by bad item pricing. I talked about it a
bit in this topic.
The problem with picking equipment in shop wisely is that you are trying to introduce customization options with a "limiter by purchase" in a game that "enables grinding for money". These two are at odds with each other. If the pricing for the items is too high to get everything, making you choose what to buy - the player can just go grind for cash. And by that - overlevel, making the future encounters too easy by sheer stat gain.
Plenty of ways to go around this, but that is a topic for a different discussion.
(if you want to - Curia can explain why we have different currencies in the world and how to implement that logic into a game)
As I tried writing out the reasons, I noticed it would end up too complicated and more confusing in the end... Sorry, I will have to skip this part.
Let's just talk a bit about game implementation and problems in games.
You see, the first question you should ask is "Does my game gain anything from this mechanic outside of flavor?" You do this, because Strawberry Frustration will not really be better than just Frustration caused by a game mechanic. In most games - multiple currencies are simply a USELESS feature.
Now, if your game is about being a Merchant that travels the world through different countries and choosing what to buy and what to sell in a given country to acquire their currency, selling when its high and buying when its low but will be coming back - you create an interesting mechanic. "If people need Wood - Should buy from X country or Y country? X has a higher tax rate, but I have no money in Y, yet the Z country will buy any wood I got for a hefty price...."
On a different side - a Harvest Moon type game where you could choose where you sell the grown vegetables to what country is also a game enhancing choice. "X country pays more for Corn, but they only sell Fall seeds. Y has a lower price on Corn, but if I sell to them, I will have currency to buy my Summer Vegetables and that Double bed...."
Now - these two games share one common trait - you can acquire each currency at any time.
What about RPGs? If your RPG is standard and has a storyline where you travel, you will get frustrated by different currencies, since once you are somewhere - you are stuck there with their shops and currencies. If he cannot shop for something you need there - the player will be annoyed. If he has to GRIND to get stuff - the player will be annoyed. If he has money he cannot use OR gets taxed because of exchange rates between countries - the player will be annoyed.
That's why games go mostly for Gold or some other global value for all countries instead of USD, PLN, EUR, GBP, CAD, AUD, etc.
If you need some extra resource akin to gold - its mostly to limit what you buy from a certain group of items. For example, you can have Gold for standard equipment and "Gems" to buy special accessories. Having gold shops everywhere but Gem shops only in select places will let you limit the access to these special items. Its used as a balancing tool this way.
On the other hand - try reversing the question. "What multiple currencies give me in my game?" If the only answer is "flavor" or "realism" - do not attempt adding them unless your game is a Simulator. Adding any kind of feature into a game will take you time and effort + it can create problems. Unless you are sure you need the feature, it is generally better to skip it.
I know this might sound like I am against multiple currencies. I don't want you to feel its the case though. I am mainly asking for the reason you would want them in the game. If they serve some purpose, implementing them might be a good choice. But if you are thinking "it would just be cool no?" then the answer would be "if you will purposefully annoy the player by it - no". Some games are just not fit for certain features.