I disagree that socialism is just an economic system. The ideology is also anti-cultural: it has no religion (or even declares war to religion itself), destroys the culture and tries to erase history; all of this can't subsist with nationalism.
You are thinking of Leninism or something like that, maybe Trotskyism. A country can be socialist and have a government faith (f.e. Norway), and it can have an offical tongue, and governmental holidays (based on cultural traditions), and all this, Norway has. Socialism can stay in one country, it does not have to be worldly, and thus nationalism (love of your country), I see no why not.