Edit: Apart from language, I can't think of any other cultural attribute which is strong enough to define a nation.
If you'd consider the Dutch and Flemish to have enough difference to define nations, then what about other regions of the Netherlands, or the different counties of the UK?
You now seem to be diving into their examples to, in essence, agree with them: language itself is not a good common property to define a nation.
Coming from Serbia and speaking Serbian (but not Croatian or Bosnian, though we'd understand each other as if we are speaking the same language ;)), even language is a term used for political purposes, but what you make of it in relation to defining a "nation" is up to you.
Although, personally, I'd find language to be the only thing there that I'd care about
States speak of administration; nations speak of common bonds. States seek to turn their diverse populations into a common nation because it will create stability. But when some of the people who are subject to your administration feel a common bond with hundreds of millions of people on the other side of a somewhat arbitrary line, this may be an impossible or undesireable task.
The idea of nationalism, that states and nations should coincide, was only partly implemented. There are numerous groups, including indigenous peoples of the Americas and Pacific or the numerous groups of India which never obtained their own states.
Some of these elements are indeed needed but not all them at once. There is the flagrant example of Alsatians who chose in 17th century to integrate the kingdom of France despite speaking a Germanic dialect over a religious matter (they were Catholics and German speaking people Protestants).
While I agree nation is a made up concept, for many a nation it goes back a long time, and it did usually start by differentiating by language (and religion was quickly added into the mix esp in the area). Still, the fact that we can track movement of "nations" between states (eg. large number of Serbs moved to Hapsburg monarchy as Ottomans invaded "Serbia", basically becoming a majority there, thus leading to seccesion movements that led to WW1), means that it can and has been defined for quite a while.
While German nation was practically defined as Prussia led unification efforts, there was a concept of Germanic tribes a long time ago.
And yes, language was the main denominator, but as civilizations developed, more was added into the mix. After all, language was certainly territorial to begin with, so it is a derived attribute by itself.
There is a province called Friesland (Fryslan) with its own language, I think they can qualify as a nation.
Language is just as fraught politically as anything else.