R David Cohen's thesis on the origin of Yiddish is limited by lack of evidence.

R. David Cohen has an interesting work (אברהם יגל יצחק ירנן) with the thesis that the emergence of Judaic languages like Yiddish and Ladino can be explained in light of passages in rabbinic literature like the following:

תלמוד ירושלמי מסכת שבת פרק א הלכה ד
מה דתני רשב"י בו ביום גזרו על...ועל לשונן

"That which R. Shim`on ben Yohai has taught: 'On that day they made a decree against [list of things]...and their language."

This speaks of a rabbinic decree opposing non-Jewish language (in a list of prohibitions including things like bread, cheese, wine, etc.).

In support he cites

שו"ת חתם סופר חלק ד (אה"ע ב) סימן יא
ולדעתי גם הקדמוני' היו בקיאי' ללועזי' בלע"ז אך בכוונה שבשו הלשונות מפני גזירת י"ח דבר שבירושלמי פ"ק דשבת דפוס וויען דף וי"ו /ו'/ ע"א ועל לשונם יע"ש ואין להאריך בזה בעו"ה

This is an attempt to account for this phenomenon. The Chasam Sofer opines that the 'correct' vernacular was known to those of earlier generations. However, they deliberately distorted it in their use of it in speech, in line with the Yerushalmi quoted above.

Indeed, the possibility of people knowing how to wield a 'proper' and an ethnic dialect of the same language at the same time is well documented and needs no elaboration here.

Thus, R. Cohen's thesis that the emergence of Yiddish and Ladino can be explained in this fashion. I believe what you have here is a classic syllogism, which is to say, that the juxtaposition of two facts (premises) are made to produce a third, previously unknown fact. In this case, the facts are

1. Yiddish and Ladino are different from German and Spanish
2. There is a rabbinic prohibition on the use of non-Jewish languages

and the result is

3. The rabbis deliberately spoke and promulgated distorted versions of the local vernaculars, and thereby is explained the emergence of Yiddish and Ladino.

Whether or not this is true is a historical question cannot be resolved by linking a 'fact' (this belongs in quotes, as I shall explain) with another fact.

The problems:

While Yiddish is different from German (and Ladino from Spanish), how different was Yiddish from the German of the time of its inception? To what extent has the development of Yiddish in non-German speaking lands influenced its divergence from German? To what extent has non Germanic languages influenced Yiddish? What of the status of dialects within a single geographical area 1000 years ago? Apparently persons living in European villages speaking dialects cultivated locally were often unable to understand people who spoke different dialects of the same language only one hundred miles away. Which German did Yiddish derive from and diverge from? What about the relationship between Ladino and Spanish? Between other Judeo dialects? Babylonian Aramaic? Palestinian Aramaic? What of Rashi's use of 'לע"ז,' which happens to be one of the most important extant source for Old French? Presumably these correct French words were being addressed to Jews for whom it was the vernacular. What of Judeo-Arabic? Suppose some counter examples are shown; perhaps only Yiddish and German (intentionally) adhered to the prohibition in the Yerushami?

These are all questions which can be answered only by examining old German literature and old Yiddish literature (actually, Yiddish literature makes a late appearance relative to the time which it probably emerged) as well as other materials which address the state of language in the Middle Ages. For example, while it is true that the German which is presently spoken is quite different from Yiddish, Yiddish itself is not so different from the strata known as Middle High German, which is the German that Yiddish emerged from, just as modern German emerged from it. That's why I placed 'fact' in inverted commas above.

However, R. Cohen's study ignores these various pieces of evidence which may be marshaled to prove or disprove the thesis. And it is a pity, because there is an enormous (even if insufficient) body of evidence to examine. But by eschewing it in favor of an interpretation of a Yerushalmi, combined with an apparent fact and supported by a Chasam Sofer (who failed to show that the 'correct' vernaculars were available all along) may have resulted in the historical truth of the emergence of Judeo languages, but it also might not have.

Indeed, for the second half of Yiddish's history, when it migrated eastward from Germany, it is an apparent fact that unlike the other places where Jews lived they did not adopt the local vernaculars, but continued speaking their own (until the well documented struggles over this issue in the 18th and 19th century). This might well be a fact worth examining in partial support of R. Cohen's thesis. It would not show that such languages were deliberately created from on high for the masses (can such a thing be? Maybe--`Ivrit?) but it might show that a conscious effort to adhere to the prohibition in the Yerushalmi. On the other hand, other factors, such as the ghettoization of the population might be the cause.

This particular thesis was cited in this letter in Jewish Action. I think there is something of a shame that both the writer of this letter and the author of the work that was the subject of this post are brilliant. But a question such as this can't be resolved by ignoring all the available evidence in favor of syllogism. Indeed, sometimes all there is is thin evidence and the only way to arrive at a working theory is through exegesis. If the only evidence appertaining to the rise of Yiddish and Ladino were the Yerushalmi and the Chasam Sofer and a few slivers of other sources I'd understand and accept such a theory.

Logical inferences are very useful in producing important new thoughts, but we all know that sitting in one's study and logically proving that the planets and stars must be attached to concentric spheres is ultimately self-limiting when one can also gaze into a telescope.