content warning: long post on the topic of nationality. take care of your organs of perception.
"prove that you're [insert nationality here]"
national identity is not something you need to prove. you cannot "prove" your nationality even with legal citizenship, it isn't the same. cultural heritage, language and traditions often learned and experienced rather than formally documented.
for example, what is suggested to do for those who belong to other country due to migration, sometimes involuntary? for those whose nationality was forcibly incorporated into the specific state during the process of colonisation? for those who living in diaspora and have a complex, hybrid identity that is a blend of their heritage and the culture of the country they now reside in?
"you aren't [nationality]! being a part of [nationality] means it: ..."
any national identity isn't supposed to be rigid. cultures also aren't static. it's a social construct. it's shaped by a complex interplay of political, sociocultural and historical factors. the fact of their dynamic nature means that defining a "pure, authentic" version of any nationality or culture is inherently troublesome, contentious and harmful. it's constantly evolving. if it is not, it suggests view at national identity as biologically determined and leads to essentialist thinking where someone judged based on assumed traits of their nationality.
any identity, not only national, is created and maintained thought media, education and social institutions. you can create your own identity, any number of them, social constructivism is a funny thing. you can express your attitude to the concept of nationality as something indefinable by standard country-related concepts by identifying with xenonationality.
just remember that each label comes with a set of expectations. if you talk about your nationality, you will be expected to know the language, traditions, etiquette etc.
that's all i wanted to say. have a nice day.