This is a rather complicated and vague question because I have forgotten a lot about the circumstances where I learned about the language in question, specifically I have neither the reference book nor the name of the language.
As far as I remember, the language is question was described as a language that is typologically markedly different from Chinese. It was spoken by 10.000 families at one place (village or town) completely surrounded by speakers of Chinese.
The book where I read about it was definitely published before the 1980ies and was (probably) published in German (either a German original or a translation from another language). It was about the languages of the world and also explained some basic typological features and language families. It was at a popular science or introductory level. It was worth having in a university library, I read it in the reading room of Mainz University library.
I'd like to know: The name of a language fitting to that description, if possible the author and the title of the book where I might have read about it, and an answer to the question why this language does not appeared on lists of isolated languages.
P.S. I have read this question and its answers but nothing there rings a bell.