"Truly bilingual (and multi-lingual) ethnic groups
are rather rare appearances, but even they exist in known history. Thus,
it is not at all a given fact that throughout known history, ethnicity
and language did usually go hand in hand, even that they did often go
hand in hand is very disputeable."
I would like to put forth the history of Jewish Diaspora here. It is true
that it is probably a very unique example, but whereever Jews went in their
migrations, they not only preserved their language and culture, but also
learned the language(s) of their adoptive countries, sometimes even
creating new languages - amalgamations of German and Hebrew which was
Yiddish, a variation of Spanish which was Ladino, and in Morocco, a
combination of Spanish, Hebrew, and Arabic which was called Haqetia. There
may be more such examples of which I am ignorant, but would this not be an
example of bilingual and multi-lingual ethnic groups? Also, my people on my
father's side fled the Spanish Inquisition and migrated to Morocco in the
late 15th Century (1492). Spanish has remained the everyday spoken language
for these people to this day! They learned Arabic, they learned French
(because of colonial presence/control), but they continued to speak Spanish.
And I, the half-American hybrid, learned those languages along with English!