A practical way up to deal with this is to (1) use the City field to enter both the city and foreign postal code and (2) use the Zip Code field for ISO 2-letter country code.
Here's an example:
Street Address: 123 Main Street
City: London ABC1 9DE
Zip Code: GB
On Schedule A, column B, this is how it will be printed:
123 Main Street London ABC1 9DE, GB
I wouldn't worry about the diagnostic since the return will be filed on paper anyway.
