If the principal authorship or institution cannot be determined for some reason, then the country name alone can be used. In actual practice this is rarely necessary. The first-name precedence cited above should be used if the Corporate Source is in different countries. However, a choice of one country must be made. The location of the conference is strictly incidental.
Groups move their conferences around for a variety of reasons.
More fundamental is the location where the author resided when the paper was prepared. If the GEO choice is between these two, choose the author's location. If all that is known is the conference location, then use that. The location of the author cited on the document at the time a document was prepared should be cataloged. Do not use the author's new location unless the original location cannot be determined.