United States is a nation, in so much as any nation is a nation. Countries in the Americas are notoriously divergent from their predecessors in Europe. Every country in the Americas is and/or was a young immigrant nation. Indeed, such is how they were populated and these populations arose over the past 300+ years, with the bulk of such happening over the past 150 years.
Language is an old card, but an invalid one. Russia has 27 "official" languages, with over 100 other minor languages being spoken in various regions/republics. 12% of Russians speak English. There's also German, French & Turkish footprints. Spain has 4 official languages (one primary, 3 co-): Spanish, Catalan, Galician, Basque, Aranese. Then there's Aragonese, Asturian, Leonese, and many other languages and dialects. Italy has 12 officially recognized languages, with 27 total regional languages, etc. And then there's the U.K., with English, Welsh, Scottish Gaelic, Lowland Scots, Cornish, Irish. Other notable languages in the U.K. are French, German, and Spanish, with 17 other languages posing their footprints.
The Americas are in a unique spot, between the firmly historic western and eastern nations. As such, both European and Asian influences abound, with Brazil having the largest Japanese population outside of their origin nation. The argument about culture dwells mainly on the notion that a region has a history, and with it certain rituals. Unfortunately (or perhaps fortunately), modernized regions have have largely succumbed to cultural cosmopolitanism. Such as it is, culture is a thing of the past and not necessarily border-defined. That's not to say it no longer exists, quite the contrary. Cultures thrive, but they are largely driven by underdevelopment and community dependence.
As to borders, oh there's no doubt as to where the borders are --- just look for the paranoia and societal schisms. Borders define nations, but also tend to create regional conflicts that are largely fueled by economic divides. Border issues, including the territorialism associated with hostility towards immigration, is mainly the result of thriving economies fearing that those from poorer economies will somehow undermine their own economies (take jobs, increase cost of living, reduce pay, increase crime). This has been firmly disputed by experts in economics and many social studies have demonstrated that financial insecurity is a major factor in driving unwarranted hostilities towards immigrants.
But, really, is any nation truly a nation? What constitutes a "nation?" That is the underlying question here, despite it being posed towards the United States.