In Capitally, any asset can turn into a liability - its quantity has to go negative. You can either "Buy" assets by setting a negative quantity or "Sell" them first with a positive one.
Create a Custom Asset, choose Loan or Mortgage for an asset type.
Create a regular Buy transaction when you've taken the loan.
Quantity is negative - equal to the amount you borrowed
Price is simply 1
Create a Sell transaction when you repay the debt
Paid principal is your Quantity - again it has to be negative
Paid interest can be your Fee , or you can log a separate Interest transaction with a negative value (because you're paying the bank not the other way round)
You can easily create an import template to add principal & interest payments in bulk from your broker or spreadsheet.
Use Accounts or Tags to group loans together with what they finance (eg. Real-estate + Mortgage, Stocks + Margin account).
The Market Value tab has an Owed chart and displays three relevant metrics:
Owned - how much value you actually own (your assets)
Owed - how much value you've borrowed (your liabilities)
Debt Ratio - how deep you're in debt (as owed/owned)