> During the colonial period, European powers invading and occupying countries in Africa, Asia, and the Americas was a common occurrence, and these actions were often met with little or no condemnation from other countries or international organizations. This was due in large part to the prevailing belief at the time that European nations were culturally and technologically superior to the peoples they colonized, and that it was therefore justifiable to bring these "uncivilized" peoples under European control.

Just to add to that, Chinese and Japanese colonialism followed the exact same dynamic, right down to justification based on the idea of their own superiority.

Muslim colonialism was a massively long-term enterprise, but obviously relied on the belief in the absolute truth of Islaam for its justification. That said, I'm not aware of any questioning from within of the Muslim expansion. None. Because thwt would likely be seen as blasphemy.

It's hard to determine how Inca colonialism was justified. It started in southern Peru and eventually conquered lands from southern Colombia to south-central Chile. It often made use of Stalinesque mass population transfers to prevent rebellions and insurgencies. But as to justifications, there wasn't necessarily one at all. Expansion and conquest seem to be the default human group behavior, and thus require no more justification than eating, except for civilizations with an extraordinarily highly developed sense of ethics, as these are the only ones que question such obviously beneficial actions as conquering more lands and peoples.

> Let's assume this theory is correct

It is not. It is a crackpot conspiracy theory.

> ...and that there never was an invasion or mass migration of Germanic peoples to the British Isles in the early medieval period.

Why would you do this? It is as dumb as assuming there was never an invasion or mass migration of Italic peoples (the Romans) into Gaul or Hispania.

> In that case, how the hell did a new Germanic language spring up on an island that had previously only spoken variations of Latin and Celtic?

There are only two possibilities - either the Germanic language we know as English isn't actually spoken in Britain, or the Angle-Saxon-Jute denialists are cranks.

Hint: English is spoken in Britain.