Nationalism is the last bastion for people who feel that they belong to their country, their language and their culture. Borders are essential for a nation. The border runs where the languages differ. National language is therefore the natural barrier that defines national borders. Conflicts happen at these contact spots.
Conversely, without this definition and border sovereignty, there would be no foundation for a nation. If nationalism is not a given, and since ‘the whole world’ defies a precise definition, the only fall back is to relate to ones nation. A person cannot be a citizen of the world because one wouldn't know which identity would be ones own.
German people relate to German people, whether in speech or in person, by principle.
This principle is the foundation, and the exceptions to this principle only confirm it.
When you turn this principle upside down, it would express "German people relate to non German people, by principle". But this makes no sense at all. Since if you, as a German, relate to non-Germans, by principle, what makes you German in the first place?
(Replace your nation with mine when reading)