Stephen got part of it right but got a huge part wrong. He is right the interest won't survive in court; It is clearly usurious so that will be struck down quickly. HE is also correct that courts will award damages in USD (legal tender). That doesn't mean Pirate can pay the USD equivalent of when he borrowed. He did borrow BTC (no different than borrowing gold, or say a stock). When paying in legal tender to satisfy a debt owed in other assets (gold, futures contracts, borrowed & shorted stock shares, BTC, etc) the courts would view anything other that repayment at current fair market value as insufficient to satisfy the debt. Of course the debtor can accept a reduced payment but that is a different story.
So if I owe you 1 oz of gold I can satisfy that debt in dollars (even if you hate dollars). This is often confusing to some people ("No you have to pay me in gold"). The reality is I can't be legally obligated to give you gold (more on why in the last paragraph), but I am obligated to pay a fair market value. What is fair market value well that is open to interpretation but most likely the courts would see repayment at current spot price plus some margin to cover the replacement cost (coinage cost, shipping, etc) to be "fair". I can't say "Here is one dollar. It says good for all debts public and private so I no longer owe you that ounce of gold".
A more complex hypothetical example (item can't be replaced):
I borrow your motorcycle and total it. Oops. You sue me for loss of the vehicle. The courts obviously can't make me return the totaled motorcycle back to you in the prior condition (i.e. rebuild it at the atomic level back into the pre-crash state). The courts can't even make me give you another motorcycle. The courts however can award DAMAGES for the VALUE of a motorcycle, and what form would those damages be expressed in? Why legal tender of course. So you bring in a bill of sale for a new replacement motorcycle and the bill of sale says $18,523.87 the courts could award you damages in that amount. You could then use those funds to buy a new motorcycle or not. The courts don't really care. Courts only care about the VALUE of a loss and award damages based on the value.
How this would relate to Pirate:
First I don't think this is going to court but for the sake of the argument say a judge found that Pirate owed a plaintiff(s) x BTC. A judge wouldn't award damages in BTC (it isn't legal tender). Plus it creates all kind of difficulties the court doesn't really want to deal with. Pirate "might" (snicker snicker) not have the BTC. It might not be obtainable (maybe he owes more BTC than exist or are available for sale). There may be technical or legal restrictions which prevent him from getting the BTC. This is why in the US (and in US courts) the dollar is "good for all debts public & private". A judge (hypothetically speaking) would award damages in USD based on the value of the compensatable loss. As an example a judge could find Pirate owes 1,000 BTC and agree with the plaintiffs assertion that the current fair market value is currently ~$11 per BTC (no the judge isn't going to go tick by tick) and award $11,000.00 in damages, then levy a judgement for that amount against Pirate.
Now this doesn't mean all debt have to be settled in dollars. If both parties agree to settle the $11,000.00 in BTC, LTC, gold doubloons, or even in metric tons of manure that is perfectly fine. However if the parties can't reach an agreement as far as the courts is concerned the original terms, and assets, and contacts have been replaced with a $11,000.00 judgment in LEGAL TENDER.
One last wrinkle. This means if for some reason (once again hypothetically and it is hard not to laugh when saying it) Pirate had ~$5M USD but didn't have the ~500K BTC. He wouldn't have to buy the BTC. He could pay your BTC debt in USD equivalent. It actually would be smart to do so (no slippage or fees). As long as he gave you a fair price (like spot price at time of repayment) creditors would have no recourse. Why? Your only (legal) recourse would be to sue him in court. As discussed above even if you win the court is going to award damages in legal tender (USD).
WHEW!
This is an excellent analysis. There is another wrinkle here. What if the debtor has publicly admitted to the manipulation of the market for the asset in which the debt was denominated?