Here in the Southwest Spanish has been part of our culture (White Americans, that is) for a century or more. I see nothing wrong with having another language present in our country. It has been common in certain Asiatic and European countries for a long time. No one, except possibly the French, seem to mind. I don't mind either. Most of our landmarks are already named after Hispanic or Native American people, events or places. It does not diminish America.
I was going to simply give you a heart, but then I did want to say that the integration... sometimes less kind than other times... of different peoples has been the core of the American nation. 250 years ago, the alliance of colonialists with the French allowed our independence; that was a time when nations in Europe could be truly mortal enemies.
Nonconformists or dissidents from England, Holland, France and other European nations found an opportunity in America. After the War of Succession, Germans, Italians as well as more Irish, British and other Europeans fled the conflicts and poverty in their homeland.
In the meantime, Swedes and Norwegians quietly populated the upper Midwest where the winters made them feel right at home and the fertile lands were vastly more productive.
After The Great War, displaced Europeans of all nations began arriving with huge Polish communities being examples of that migratory period.
Jews from all over Europe found, despite original rejections, refuge in the US.
And all along, the involuntary relocation of prisoners from tribal warfare and conquests in Subsaharan Africa brought millions of people to the US as slaves. And the expansion of territories to include Florida and much of the West and Southwest brought a Spanish influence and culture to the mix.
Native Americans were all to often seen as an annoyance, to be shuffled around the new nation at the convenience of the conquerors. While those had been the common and normal procedures of conquest going back to earliest societies, it certainly invalidated the idea of America being a land of freedom for all.
As you say, accepting the Spanish and Mexican cultures in the Southwest does not diminish the national character. But the fact that, still, many object to this and are openly bigoted and biased, shows that we are not there yet. But what other nation, other than our neighbor Canada, has ever tried to thrive with a never-ending blending of cultures, races and beliefs?