If you have ever been to Europe you will notice people speak alot of languages. Ive seen people there 7 languages. The jews were no differnce.
Cities Like Prague, Bratislava (Pressburg) , Krakow, Lviv (Lemberg) , Budapest jews probably spoke a bunch of languages German, Polish, Czech, Slovak, HUngarian, Ukranian, Russian and probably others as well. it made a lot of sense to have a language frica Yiddish, People spoke the regular language and also spoke yiddish. Yiddish was the common language. This is not the case anymore.
Lior there are some who want to do that to English, they call it Yeshivish. I am not convienced its much differnet than English though