Sadly, it's not that simple!
Suppose you in the year 2013 have 200k units of Foreign Currency X (FCX) in your non-US savings account, and the money has never had anything to do with the US (it's all been earned and saved abroad, where you live and work). Let's say the conversion rate in 2013 is 2 FCX to 1 USD. OK, so you report 100k USD on your FBAR. Let's say 2013 is a bad year for you, so you haven't been able to put anything in your savings account - in 2014 it still stands at 200k FCX. But lo and behold, the value of the USD has fallen against the FCX, so the dollar equivalent in your account has now in 2014 become 150k USD. This means [1] that you have to put 150k USD on your FBAR and that the difference of 50k USD is taxable as capital gains. This is utterly, entirely, completely insane! It's nothing short of international robbery. (I repeat: I'm not talking about money earned in or brought into or out of the US here - I would understand if it were taxed. This money has never had anything to do with the US, and has never been converted from one currency to another, except on the FBAR form.)
This cannot be OK. Not at all. I understand that the laws were made for catching US money hid away overseas. That goal makes perfect sense. But the way it works now is just insane.
Second, the US does not tax unrealized capital gains. In the situation you have just described, you have not realized any capital gains and have no taxable income, even if you believe that receiving currency in year N and disbursing of it in year N+X counts as capital gains if the currency has appreciated against the dollar. This interpretation is contrary to what the IRS will tell you is the law.
Should you want to actually use the money, you should say that "My cost basis in the 200k Euros is 200k Euros. I liquidated them at their fair market value, of 200k Euros. This gives me a capital gain of 0 Euros, which when I put it on forms I will convert into dollars, for capital gains of $0." There are more complicated things you'll want to do if you operate a currency exchange business, but that isn't relevant to you.
If you disagree that this is the correct interpretation of US tax law, you can do what I did and phone the IRS. They'll explain it to you.
If you doubt this is correct, use some of the 200k Euros to have a brief chat with a tax accountant. I know this sounds new and scary for you. It was new and scary for me, too, eight years ago. It's a lot easier than you think it is.
For a US citizen, the functional currency for personal taxation is ALWAYS the US dollar. See particularly points 12 and 13 of http://openjurist.org/93/f3d/26/quijano-v-united-states
Basis must always be converted to US dollars using the exchange rate at the time of purchase, and sale price likewise converted to US dollars using the exchange rate at the time of sale. The US capital gain is then the difference between these two US dollar values.
This can easily (and frequently does) lead to having to pay a capital gain on a transaction that lost money in the currency in which it actually took place. Particularly the last decade was bad for this with the falling US dollar.
Many expats (if they are aware of their obligation to worldwide taxation which most are not) may calculate this incorrectly. This will often be ignored by the IRS, because they have no matching information to use to flag the foreign transaction for audit. However, with FATCA coming, the IRS will finally get some matching information and it is a strong possibility for nasty surprises in the future.
Mine took five minutes to fill out. Four minutes to find the postal address for my bank branch, one to flip through my bank statements to find the peak balance and covert it to USD.
It may get slightly more complicated if you control an account in the Cayman Islands with $30 million which you might have forgotten to file on your taxes for the last 30 years.