The striving for a linguistically homogeneous nation state in Europe is strongly associated with the French Revolution, which was one of the major expressions of the Enlightenment. It was then that a centralized government began strongly sanctioning regional languages that the monarchical regime had largely left alone (albeit out of any official use).

After that, the next big wave was the revolutions of 1848, which were inspired by national romanticism, but it’s valid to see that as an evolution of ideas that first arose in the Enlightenment. It certainly wasn’t out of any belief in royal absolutism.