If you stay in big groups and don't have to deal that much with a dominant culture, you keep your language. If you're forced into small groups and thus have to deal with the dominant culture on an individual personal basis, you better learn the lingo. Your kids will learn it better than you will at any rate.
What doesn't happen in any event is the appearance of a really new language out of this situation. And from what Indoeuropean conquerors did the Hebrews acquire their "Celtic" or whatever language?
The language question is certainly one that should be examined at some time, but to hang any substantial issue on language is IMHO just silly. Language is not like genes, it just the way in which they communicated. In the case of the Celts this was mostly verbal as their relegion or whatever frowned on leaving any written records.
In the case of the Celts, they didn't need to "acquire" their language from any conquerors. They were the conquerors, so their language probably came from somewhere else.
So which is it? Should the language have been preserved as you say, or should nanrod's "good point" be right and the language been obliterated?
What doesn't happen in any event is the appearance of a really new language out of this situation. And from what Indoeuropean conquerors did the Hebrews acquire their "Celtic" or whatever language?
Tell me all about this Celtic language and we'll see. I don't know anything about it and don't even think it's an indicator of anything that happened and the experience in America has proven.