The declaration is the first step in preventing government services to be offered in languages other than English.

English is the dominant language in the US, nobody argues that. But making it the official language has a lot of implications, both symbolic and bureaucratic.

Needless to say one of the "unintentional" side effects of this EO is the nationalist posturing, giving some ammunition to chuds who like to shout things like "speak English, this is America", and general eurocentric colonial sentiments.

It also has the possibility of introducing barriers to migration, which given the administration's expressed interest and prior action, is a guaranteed motivation. The fact of the matter is that the people coming here who are most likely to know English (thus be admitted entrance into the country and approved for citizenship following this EO) are going to be white. Having worked in higher education, this would be massively damaging to so many international students, never mind all the many other people who might be seeking entry to the country for all the myriad of reasons one might do so.

There are also significant legal ramifications for the millions of current citizens for whom English is not their primary language. The implementation of an official national language is certain to erode accessibility of bureaucratic systems to those who rely on multilingual resources, which were previously mandatory for public services to provide. Moving forward, we're very likely to see many of these resources become unmaintained (if not be scrapped altogether), while the material published in English is kept updated. Again, this is absolutely going to disproportionately impact people of color in the US, who are more likely to not speak English as a first language, or be fluent in it at all.

And while I feel like those points speak for themselves, I'd also like to add that I grew up being preached to about how great the US is for its multiculturalism. I was taught a whitewashed and distorted history, but the benefits of multiculturalism were never overstated. We benefit from being surrounded by people with experiences vastly different than our own who we can learn from. Language is intimately tied to our capacity for understanding and developing knowledge, so any attempts to suppress the presence of languages other than English block us from any growth we might experience from them, either on an individual or societal level. For so many minority groups, particularly indigenous peoples, language is one of the most raw connections we have to our history and culture.

Tl:dr (but please do read it all); designating a national language, in the US, with its current administration, is a transparently nationalist effort to harm minority groups, both prospective migrants and current citizens.

The US has no official language because it is supposed to be the land of the free, where anyone can come to escape tyranny. This is another dog whistle (siren really) that will be used by the racists, bigots, and fascists to continue a campaign against minorities and promote greater deportation.