When Republicans talk about Nationalism, they are talking about all the tribal factions that make up the US and point to them as Americans. Those who say such things are dog whistles for whites, simply say so because thats how they want to view everything.
Some people simply like to look at everything as tribal or identity politics. I am not going to subscribe to that notion. I do not and am not going to treat people differently because their not like me. For many foreigners and 1st gen immigrants I've worked with I've respected whatever traditions or customs they might have. No reason not to that I've seen.
The reason the US has become one of the richest and greatest countries is still the melting pot concept. That does not mean people can't keep whatever traditions and customs they want, but they also learn to adapt to American society. Anyone ever go to Italian festivals, or Greek or Russian or Mexican or whatever. Their usually great and people don't say oh, you can't do that anymore because its not American enough. Instead such things are celebrated. So in this case we can have our cakes and eat them too!