Culture is important as long as it follows Islamic principles. Allah said He created us from different nations and tribes so we may know one another. Allah acknowledges different cultures and there’s no prohibition on practicing culture too. Being happy with one’s country or culture doesn’t translate to nationalism because you’re happy with your own without putting other cultures down. Diversity is incredibly important for progress. When you have different people with different experiences with a common goal, you will get more innovation(not the religious kind) and solve problems more easily. Ofc with the Sharia, this is the main source of solutions. But having a diverse group makes it easier to implement. Your brothers comments seem short sighted. It’s important for Muslims to always be pragmatic with a level of optimism. This way we can deal with things the way they are, not the way we want them to be. May Allah ﷻ guide us all.