Jews often tend to adopt languages of people among whom they live (for instance, English in the USA).
Germany's Medieval Jewish community came from two main sources - one part were Jews who had settled in areas which were western parts of the Frankish Empire and later East Francia (they eventually developed the Western Yidish language), and the other part were Jews who had settled in West Slavic lands such as Bohemia, Lusatia, Polabia, Pomerania, Silesia, etc., which later became parts of the HRE during its political expansion eastward.
That latter part initially spoke Judeo-Slavic language, so-called Leshon Knaan.
Interactions between Western Yidish and Leshon Knaan led to development of early Eastern Yiddish. The last known written texts in Leshon Knaan language date back to the late 1500s, from areas then belonging to the kingdom of Poland. After that Leshon Knaan got extinct, due to increased expulsions and flight of Yiddish-speaking Jews to the east, where they mixed with Knaan-speaking Jewish communities:
https://en.wikipedia.org/wiki/Knaanic_language
That Judeo-Slavonic language was spoken not only by Jews living in Bohemia and Poland, but also by Jews living in Lusatia and areas inhabited by Polabian Slavs. But indeed out of West Slavic lands, the oldest presence of Jews was in the Kingdom of the Czechs (which was established earlier than the Kingdom of Poland), and perhaps also in lands of Lusatian Sorbs. Only later Judeo-Slavic Jews came to Poland - long after Christianization of Poland. But those were relatively small communities, and many of them converted to Christianity throughout time.
There was no numerically significant presence of Jews in Poland - at least not compared to countries such as Spain, Italy, Germany, or even much smaller Portugal, where numbers of Jews were large before expulsions - until the second half of the 16th century.
Most likely Jews settled already in Great Moravia, and were later "inherited" by the Czech Kingdom and other lands of former Great Moravia.
There were also few Jews in the Principality of Halych-Volhynia, and those Jews likely spoke Leshon Knaan. For a short period of time, Western Ukraine belonged to the early Czech Kingdom of the Premyslid dynasty. Perhaps that's when first Jews settled there.
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So Ashkenazi Jews refers to Jews speak Yiddish, or to Jews who came from Germany.
Despite being called Ashkenazi, some 90% of modern Ashkenazi Jews descend from people who - at one point in time or another - lived in the Polish-Lithuanian Commonwealth, which became the only safe refuge for Western European Jews during the pogroms and expulsions of the 1300s-1500s.
Judeo-Slavonic language got extinct in the 1500s as the result of mass immigration of Yiddish-speaking Jews from the west.
But Jews increased their numbers so much in Poland-Lithuania not only due to immigration, also due to high fertility rates.
Already in year 1598 certain Sebastian Miczyński noted the following about Jewish fertility:
"Jews marry at age 12, they don't fight in wars, they don't die from bad air, and so they breed like rabbits."
That Early Modern Era Jews had so high fertility rates is not so surprising - just look at modern Orthodox Jews in Israel. They also have very high fertility rates, even higher than Arabs. And now we must realize, that back in the Early Modern Era, virtually 100% of all Jews were Orthodox Jews.
Stanislaw Plater, a Polish noble descended from Polonised Germans of Livonia, did not have sympathy for Jews.
In his 1825 book, xenophobic Plater described the Jews of the Congress Kingdom of Poland in the following way:
"(...) Polish burghers are no longer in majority. In most of smaller towns Jews have outnumbered them by now. Many cities to which 300 years ago entrance for Jews was forbidden, now have around 50% of Jews among their population. In some other towns Jews already outnumber Christians. Wherever Jews move in, they ruin the locals with their selfish trading-monopolistic solidarity. (...) With this method, Polish cities are becoming more and more Jewish. The look of Jewish-inhabited districts, for people who know how cities look in other countries, is not very nice. (...) Only in southern part of the Congress Kingdom of Poland, in the vicinity of Kielce and Bodzentyn, few dozen towns are still free from Jews. (...) Jews, living in Poland for few centuries, now belong to the most striking phenomena of this country. In no other countries have they become so numerous, nowhere have they preserved so much of a separate nationality. Elsewhere, they are now just a religious sect, but in Poland they are a nation within a nation, which is preserving their own language, their own traditional costumes and customs. Hebrew is for them what Latin is for Catholics and Old Church Slavonic for Eastern Christians, that is - the language of religious rituals.
Their vernacular is strange and wild, a macaronic mix of German and Polish - it proves, that they came to Poland not from the east, but from the west. (...)"
Apart from immigration, Jews indeed had a higher fertility than Non-Jews. Z. Guldon and K. Krzystanek in book "Jewish population in towns of the left-bank part of the Sandomir Voivodeship in 16th-18th centuries. Settlement-demography study", Kielce 1990, wrote that percent of Jews among inhabitants of towns of Sandomir Voivodeship west of the San River increased from 7,4% by the end of the 1600s, to 22% by the end of the 1700s.
In the Congress Kingdom of Poland, which was described above by Plater, percent of Jews among the population increased from 6,3% in 1816 - shortly after the Kingdom was created - to around 15% in 1913, shortly before WW1. Not all of that growth was due to natural increase, as there was also westward immigration of Jews to Congress Poland, mostly from Belarus, Lithuania and Ukraine. Those were so called "Litvaks".
By contrast Jews from the Grand Duchy of Poznan and from Galicia emigrated westward, as the result of which percent of Jews in the Grand Duchy of Poznan decreased from 7,8% in 1816 (as you can see, in 1816 percent of Jews was higher in the Grand Duchy of Poznan, than in Congress Poland) to just 1,9% in 1900. That was due to westward emigration to Germany (including Berlin) and to the USA.
In Galicia percent of Jews among the population was also decreasing, but not as fast.
In modern Israel, Orthodox Jews are the fastest growing ethno-religious group thanks to their high fertility rates. In the Early Modern Era all Jews were Orthodox, and they were also characterized by very high rates of natural growth. That's how Poland-Lithuania became home to by far the largest Jewish diaspora in Europe at one point in time (the second largest one was in the Ottoman Empire) - due to immigration from countries from which Jews had been expelled, combined with high natural increase.
The Russian Empire had no Jewish inhabitants within its 1719 borders. Until 1762 the number of Jews there increased to 66,100 due to annexation of formerly Polish-Lithuanian Livonia, as well as some immigration.
In Prussia and Austria the same was the case. Before 1648, there existed no Jewish communities there as far as I know, all had been expelled by that time. However, some Jewish immigration from Polish-controlled Ukraine to lands controlled by the Habsburg dynasty and to Prussia-Brandenburg started already in 1648, during the rebellion by Chmiel the Wicked (as he was called by Nathan Hanover in "Abyss of Despair"). In Berlin first Jews showed up in 1671 (after one century since last group of Jews had been expelled in 1573), and first synagogue was built in 1712.
That said, the bulk of their Jews was acquired by Prussia and Austria - just like in case of Russia - in the partitions of Poland-Lithuania. Prussia acquired around 70,000 Jews in 1773, then 74,000 more Jews in 1793, and finally 76,000 Jews in the third partition (1795), but lands of the Prussian third partition were later lost, and ultimately acquired by Russia in 1815.
After 1772-1795 Jews gradually emigrated from areas acquired in the partitions, to other parts of Prussia / Germany, Austro-Hungary and Russia.
For example, in year 1850 still as much as 72,3% of all Austrian Jews lived in Galicia, while in 1900 only 66,2% lived there. Similar Jewish emigration from Polish lands acquired by Prussia to western parts of Germany and to the city of Berlin took place.
In 1670 there were no Jews living in Berlin, while in 1925 the Jewish community in Berlin numbered 173,000 people.
So vast majority if not all of ancestors of Russian, German and Austrian Jews had lived in Poland-Lithuania at some point in time between year 1647 and year 1795. During that period, Poland-Lithuania was one of perhaps only three countries in Europe - alongside the Ottoman Empire and the Netherlands - that were tolerant for Jews.
Of course there was also emigration across the ocean, to the USA. In period 1830 to 1930 in total some 3,060,000 Jews emigrated from Europe to North America, ncluding 2,900,000 just to the USA (of them 2,447,438 in period 1881 - 1932, around 1,500,000 in period 1900 - 1914, while 296,852 in period 1919 - 1925, and only 65,841 in period 1925 - 1932).
There were a few Jews in the USA also before 1830, for example in 1790 Jews were 0,25% of the U.S. population.
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In the first half of the 20th century, Jews in Poland were a group of large social contrasts.
There was both a large group of rich Jews (compared to Non-Jews) and a large Jewish underclass (compared to Non-Jewish).
In 1921 Jews (believers of Judaism) were 10.5% of the population, but as many as 22.2% of all unemployed people, 18% of all employed people who did not reveal in the census what was their exact occupation, just 0.9% of people employed in agriculture, 23.5% of people employed in industry and mining, 73.8% of all people employed in commodity trade, 13.8% of employed in currency trade, 81.4% of employed in factoring.
In 1931 Jews (believers of Judaism) were 9,8% of the population (a decrease compared to 1921), but 27.3% of urban population and 3.2% of rural population, 21% of people employed in industry and mining, 59% of employed in commodity trade (a large decrease compared to 1921), 1% of employed in agriculture, 12% of employed in communication, finally 12% of employed in various other occupations.
High Jewish unemployement (in 1921 they were 10.5% of the population but as many as 22.2% of the unemployed population) resulted from the abnormal and highly monolithic character of their occupation profile - most of them were employed in commodity trade, usually those were small-scale traders. They were reluctant to move on to other occupations, and there was simply not enough room for so many traders.
They also faced increasing competition from Christian traders, who were supported by Christian population (i.e. "Christians buy from Christians"). And instead of changing their employment profile due to that competition, many Jews continued to do what they did, and slided into poverty. There emerged a large Jewish working class (industrial and mining workers) - still not as large as Christian, though.
In the USA, where % of Jews among population was much smaller, there was enough room for "traditional Jewish occupations". In Poland there was no room for "traditional Jewish occupations", as Jews were a very large % of the population. It was only in Palestine (and later in the state of Israel), where Jews finally realized, that they needed to start working as farmers and industrial workers as well. And indeed, Jewish colonists who moved to Palestine, learned how to farm, and started to diversify their occupational profiles. As the result, they could build a normally functioning state - Israel - where Jewish people are employed in various types of occupations - unlike in the past in Europe.
Back to pre-war Poland:
Jews were - on average - better educated than Christians, but reasons for that were: 1) that most of Jews lived in urban areas and 2) that there was poor education of Christians who lived in the countryside (especially Orthodox and Greek Catholic Christians). By contrast, if we take into consideration just urban population, then urban Christians were actually better educated than urban Jews (source in the link below):
http://www.stanford.edu/~ranabr/Abramit ... aburda.pdf
From pages 4 to 6 of the paper in the link posted above:
"(...) Since during both censuses about 3/4 of Jews resided in urban areas and 3/4 of the total population lived in rural areas, a general comparison without distinction into urban and rural areas compares mostly urban Jews to mostly rural non-Jews. Population in urban areas was signicantly more literate than that in rural areas, for both the Jewish minority and the non-Jewish population. Urban Jews - even though less literate than urban non-Jews - are still more literate than rural non-Jews. Thus, in the aggregate statistics, Jews are more literate than non-Jews. (...)"
And:
"(...) Jews attained higher levels of education than non-Jews, but lower levels in urban areas (...)"
Now some pieces of data from year 1921:
Group - active in the workforce / economically inactive population
Jews - 33.9% / 66.1%
Roman Catholics - 54.4% / 45.6%
Protestants - 53.2% / 46.8%
Greek Catholics - 63.9% / 36.1%
Orthodox people - 62.5% / 37.5%
others - 57.4% / 42.6%
Christians in total - 56.5% / 43.5%
S. Pawlowski in "On Emigration of Jews from Poland" commented that in the following way:
"(...) Number of Jews active in the workforce is in general two times higher than number of Jews economically inactive, especially in trade and isurances it is almost three times higher. (...) Such a situation was in 1921, while nowadays [1937] it must be even worse. There is no doubt that the percent of economically inactive Jews is rather increasing than decreasing. (...) Out of all Jews only 1/3 make a living, while among Non-Jews 5/9. (...) Under these conditions pauperization among Jews is progressing fast, and is becoming one of main reasons of their emigration. Jews, locking and limiting themselves to just a few occupations, are losing with each day job opportunities and opportunities to make a living. (...)"
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While of those in "trade and insurances" sector (in 1921):
Group - active in the workforce / economically inactive population
Jews - 324,615 (28.6%) / 812,316 (71.4%)
Roman Catholics - 169,026 (40.6%) / 246,877 (59.4%)
Protestants - 15,309 / 21,140
Greek Catholics - 4,861 / 5,803
Orthodox people - 4,000 / 5,534
others - 955 / 1,146
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Among Pawlowski's conclusions about Jewish well-being and rates of Jewish emigration, was:
"(...)
c) The greater the percent of Jews among the total population in a given country, the higher is their emigration rate. Because with the increase of the number of Jews in a given territory compared to its total population, economical power of Jews in this territory is decreasing and their pauperization starts. Moreover, in such a country Jews are creating strong competition against themselves, which is understandable due to the monolithic nature of their occupational profile. (...) For this reason, even Jews themselves admit, that emigration of some part of Jewish people from a given country is improving the economical situation of the remaining Jews in that country.
(...)
h) Jewish migrations generally do not meet any obstacles in countries from which Jews emigrate, but do encounter serious problems in countries to which they want to immigrate. Some countries try to totally prevent Jewish immigration, or to limit its scale.
i) Even though Jewish emigrants are citizens of various states, Jewish emigration nevertheless has a national character. Jews are trying to establish main lines of emigration for their population, and are trying to organize it, or at least to support it. In this respect Jews surpass many nations, and even states. Jews have created international emigration-colonial organizations, with very strong capitals.
These organizations include:
1. ICA or PICA - Palestine Yewish Colonisation Association - its creation was possible thanks to capital of Baron Hirsch
2. OZET - Colonisation Association for Agricultural Jews
3. AGROJOINT - association created by American Jews, by so called Joint Distribution Comittee
4. KOMZET - committee for Jewish agricultural settlement in the Soviet Union
5. ORT - association for supporting Jewish colonization in Ukraine
6. IKOR - communist organization in the Soviet Union, supporting Jewish emigration and colonization
7. PROCER - emigration organization in Buenos Aires
8. HIAS - Hebrew Immigrant Aid Society of America
They are joined by many banks and special funds, including Keren Hajessot and Keren Kayemet (...) as well as associations such as Hilfsverein der deutschen Juden in Berlin, Wiener Alliance Israelite, EZRA, and others.
(...)
m) Wherever Jews plan to settle for good [i.e. Palestine], they switch from trading, crafts and industrial activities to agricultural activities. They buy the land and settle there as colonists. However, they are encountering many difficulties during this process, because - owing to their monolithic occupational structure - Jews have no skilled farmers. Moreover, Jews are a mostly urban society and they live in large groups in cities and towns, rarely in large villages. (...) A Jew as an individual colonist is a rare case.
(...)"
As for preventing Jewish immigration by various countries, indeed - after 1918 the United States of America passed laws on immigration quotas limiting the influx of Jews, as well as other Eastern European and Southern European immigrants.