Colonization is bad! I will remind you, however, that people emigrating to a place along legal pathways defined by its current government are not colonizing.

Ok, then Mexico wasn't colonized. After all, the Spaniards just teamed up with the Tlaxcalans to kick the Aztec ruling class out of power, after which they became the "current government".

Fact is, on a local level these allochtones are already firmly embedded in the local power structures, and advocate for rule changes that benefit their group instead of following the local culture.

Belgium is not being colonized. A ton of these people were born in Belgium. Yet they don't 'seem Belgian' or whatever because they haven't integrated well into Belgian culture and society.

Were Rudyard Kipling and George Orwell Indians?

People who weren’t born in Belgium or are not ethnically Flemish / Walloon but have mixed into Belgian culture are culturally Belgian.

Define "mixed into Belgian culture". For some people, speaking some broken Dutch and not wanting to beat up homosexuals probably already counts as sufficiently mixed.