You may have to write the calculation something like this:
[Country] = "USA"
OR [Country] = "CANADA"
OR [Country] = "MEXICO"
[Country] = "FRANCE"
OR [Country] = "SPAIN"
You could also use the ad hoc grouping capabilities in Tableau to create groups for countries. Depending on your needs, the calculated field will be more flexible, but the ad hoc groups are relatively easy to create.
the code I have is an example.. real calculation is little long and complex, and based on multiple fields (group cannot be used). Inlist is very basic and commonly used...Adding it to the wishlist.