Say a US citizen buys a house in the UK and takes out a £400,000 mortgage to pay for it and at the time, £400,000=$500,000 USD.

Now 3 years later, they want to refinance the £400k mortgage they owe with a different lender but the currency exchange rates have fluctuated and now £400,000=$550,000.

In the eyes of the US, they immediately owe capital gains tax on $50,000 even though nothing happened and they didn't even move out of the house.

So yes, the US charges CGT on foreign currency and equivalents.