Having a common language, means they all speak and understand that language. They have it "in common". It doesn't mean they agree. All Mandarin speakers have a common language, but they do not all agree with each other on thoughts, ideas, beliefs, etc.

"What does common language mean? Two people can't agree but both speak English. Can I say that they don't have a common language?" dose = a certain quantity of medicine, for an illness tow = to pull something heavy Having a common language doesn't mean that you therefore agree.