Shlomo Sand quotes Abraham Harkavy's claim that Jews in what's now Poland and Russia spoke Slavic languages until relatively recently, adopting Yiddish only later. But Harkavy was writing in the 19th century, and Sand doesn't provide more recent sources. Is there any up-to-date scholarship on this question?
It's fairly unlikely that this was the case. There's a cadre of scholars in a few fields, Sand among them, who like to advocate ideosyncratic theories about the origin of Ashkenazi Jews. Harkavy did too, when those theories where first being developed.
While some scholars who advocate this are actual serious scholars, it is for the most part not a broadly accepted view. To get to these sorts of theories you have to make a series of pretty bold claims, none of which has sufficient evidence to back them.
If you're interested in a linguist who does advocate such a theory against the weight of historical evidence, your best place to look is Paul Wexler. But it's important to emphasize that his work is not accepted in the linguistic consensus, which is clear if you see the number of times he cites himself in his own work (because he's building on his own theories, not linguistic consensus) and references to unlikely historical theories from that cadre of scholars I mentioned above.
You might be interested in what /u/gingerkid1234 has to say in AskHistorians Podcast #8 about Jewish languages.