With doing so, the governments around the world can then compare each other and what their country's kids lack in. This would create a more common knowledge in kids from different countries. There are no harm of tests, and they don;t even cost anything, so why not have it... ?
It's enough that we have standardized tests that have little to do with real world skills and intelligence. And they only match up to those that study specifically for those tests. This can separate people between those that are good at solving the tests and those that aren't but are not in anyway less intelligent. This can also be used as a model of worldwide propaganda in some hands.
Different people speak differently and things get lost in translation. Even things not involved with the skill or subject being tested will influence results. For instance there was an autistic kid who tested in the intellectually disabled range of IQ at first. They looked over the test and found many questions used examples involving farm animals. Then they exposed the kid to farm animals on a field trip. Then when he was retested he got above average.
While autism and different culture aren't the same the same sort of barrier is involved. People in different cultures have divergent experiences and interests and so the examples used will influence the score.
This is even a problem already with poor and minority neighborhoods because of differences in culture. Differences in speaking style also factors in to how quickly and accurately people grasp the meaning of questions.
This may be a good idea but cultural and linguistic differences have to be addressed if we're going to have an accurate international test.