Yes, it is deductible. You can take an interest deduction for a foreign mortgage if the property is either your primary residence or a secondary residence. It is deductible, in the year of payment. You won't have to send proof of the mortgage with your return, but you need to produce the documentation in case the IRS ask  to substantiate it.

 You would convert to US currency for the interest paid as of the date of the payments.

Just remember to report the interest portion only and in USD. Enter the information as you have received a Form 1098 in Schedule A.