Hi there,
Thanks for posting, and that’s a good question. Goodbudget works best when tracking only one currency. Because of that, I’d recommend using a separate GB Household to track that secondary currency. You could add an income to that Household equal to the amount you plan to spend there (but converted to the correct currency), and then add transactions as normal. That would require you to add transactions in two places because you’d need to keep your main Household up to date too.
If you want a really easy option, then you can try doing what my husband and I did the last time we traveled. We went to the ATM, expensed a large sum of cash (which was then recorded in GB), and then we just spent the cash and didn’t bother tracking each individual transaction since the total sum was already expensed.
Hope these options are helpful!