There is no gain to be recognized for "interest rate".  Even if your client had a mortgage below market rate (which is unlikely), §7872 does not work this way, although it may trigger a reporting and information return filing requirement under §6039F.

There may, however, be a foreign currency gain that needs to be recognized pursuant to §988 on a foreign-currency denominated mortgage.  This gain is computed with each principal repayment and not only at the sale of the home.  This must also be figured separately from any gain the taxpayer may need to recognize for the sale of home and §121 does not differentiate whether a home is in the US or a foreign country.

Foreign currency gain is reported on Line 21 whereas the sale of home is reported on F.8949.

See also these two threads for discussions about foreign currency gain on mortgage.

https://proconnect.intuit.com/community/tax-talk/discussion/re-foreign-exchange-gains/01/76964#M2452

https://proconnect.intuit.com/community/tax-talk/discussion/re-foreign-exchange-gains/01/77459#M2501