Japanese-American speak a whole different language family than the Japanese in Japan. It doesn't mean that their are not gentically descended from the people of Japan. People adopt the ways and language of the people that surround them, especially a people with no roots. Your argument is senseless.
The Japanese-Americans have been largely absorbed into a dominant pre-existing culture and have adopted its language. It would be misleading to say that the American culture is the cultural descendant of the Japanese culture which they left. The American culture has absorbed a lot of groups and been through a lot of history. It is the synthesis of all that.
What I'm reading on this thread, that some group called "the Lost Tribes of Israel" later emerged as "the Celts," describes some different process, a story which no one has told but the outline of which makes no sense. To say you emerge "as" some other group means that the other group did not exist until you became them.
That's where the putative language shift looks bad. If you aren't absorbed by some pre-existing group, you don't lose your language. If you are so absorbed, the "new" culture is actually pre-existing and the new language is the dominant culture's language. If you get taken as a captive into Egypt, your new language is Egyptian, not Celtic.
You can say that some group migrated from A to B and was completely absorbed. That basically requires no evidence as it wouldn't be expected to leave much if any. But it's also a non-event. Culture B is there in any case and probably little changed.
I also note that the term of art "the Celts"--as used on this thread by LostTribe at least--includes groups that I've never ever seen identified as Celts anywhere else. This whole thing is so askew with any version of history known outside this thread that I hardly know why I bother.