I agree language and identity are tricky subjects, so I did try to be precise with my wording - language is not absolutely decisive, but I would argue on a practical level it is the most significant of a mix of factors that influence how people identify themselves, particularly in Europe.

As for Belgium, that issue is so wild that I devoted a whole separate piece to it. If you're interested: https://sjquillen.medium.com/straight-outta-fourons-belgiums-linguistic-insanity-5a7d3bce1966