I'm not quite sure what you're searching for.
You could:
a) make a separate table with all the city names and the appropriate country for each. Then when you make a lookup field in your main table for the city, include the country column in the combo box and you can populate a field with the value, which will always be correct even if Paris gets annexed by Berlin. Just change the country code in the lookup table.
b) Use update queries that include multiple criteria (I think you can have up to 8 in any one query) to input the correct country code for a given set of cities. As you say this is a bit more time consuming.
c) Use code to loop through your recordset and populate your (presumably new?) country field.
Does that help?
David R