No, every country should not have the same language, because a language is an expression of culture. Languages are interesting. They are fun to speak and fun to study. Also, the last time them entire world had the same language, at the tower of Babel, that didn't work out so well. We should appreciate differences, such as languages.
Just like animal species, languages evolve all the time. Even if all of the world's countries currently had the exact same language, a few generations would suffice to give way to linguistic variants that would not be mutually intelligible. Consequently, having a universal language is a concept that is untenable.