I like this answer because it's relatable to today's English. English is spoken by many people around the world, but it's not one group of people, it's many many different people and cultures who have come to use English for one reason or another.
The debate about 'The Celts' is that we can't point out where the original language emerged, and the potential range of where it emerged is quite huge stretching from Ireland through central Europe with some claiming it emerged in Turkey even.
The point being made is how celts are peoples that are grouped by language, but have a disparate set of histories.
And I am recognising that a contemporary analogy is how English speakers around the world also share a language, but have disparate histories.
Your problem of where the Celts originated from is separate.
Pretty sure that singular reason was colonialism.
I've merely pointed out that it is widespread and that there are (at least) three reasons for people to speak it.
Edit: Note, if you don't wish to speak it, that's fine by me - I'll be just as happy to not hear from you again :-)
https://en.wikipedia.org/wiki/English-only_movement
There are libraries of books written solely on this subject. Your bizarre hypothesis ignores this and instead points to slavery and trade. More to the point, there are new articles on how English was enforced on subjugated people published on an almost weekly basis. Last week, the article I read on this topic was about how indigenous orphans were prohibited from using their native language and forced to use only English. But let’s talk about glowing myths about trade and slavery instead which have literally nothing to do with it.
Let's have a korero in Maaori instead.
"During the period of colonization…Maori were banned from speaking their native language in public places including schools, and forced to speak the foreign language of English…Maori were deprived…of their language, but also of the dimensions of culture and history inherent in language customers and worldview."
This only changed in the 1970s.