Okay kiddo, so you know how there are different countries in Europe like England, France, and Germany, right? Well, sometimes those countries work together to make decisions that can affect them all, like how to trade with other countries or what kind of laws they should have. When they work together like this, it's called a "union," kind of like a big group of friends working together.
Now, there are also different groups of people within each of those countries who have their own languages, cultures, and traditions. For example, in Spain there are people who speak Basque, Galician, and Catalan instead of Spanish. In other countries, there are people who speak German, French, or Italian as their first language instead of the majority language. People who identify strongly with these different groups are sometimes called "nationalities."
So, imagine if all of the different nationalities in Europe came together to form their own union, where they could work together and make decisions that affect them all. This would be called a "federal union of European nationalities." It could help each group preserve and celebrate their own culture and language, while also finding common ground and working together for the good of everyone.
It's kind of like how you and your friends might all really like different things, but you can still be friends and play together and have fun, right? The federal union of European nationalities would be kind of like that, except on a much bigger scale.