The Radak (Sefer HaMichlol, introduction) writes that Loshon HaKodesh is all but forgotten to us, and all we have left is what is in Tanach.
The Chasam Sofer notes that while Chazal used many words and phrases borrowed from the Greeks and Romans, they never coined a new word, as has been done in modern Hebrew, for in their holy opinion it was preferable to use other languages rather than create even a single new word that did not have its like, its example, in the Torah, since it could not be rooted in sanctity.
The Chasam Sofer EH 2:11 says that in ancient times Jews used to use a modified version of the non-Jewish languages for everyday (divrei chol) talk, similar to what Yiddish is.
The Chasam Sofer writes that the reason Jews do not speak Loshon Hakodesh as a speaking language is because it is inappropriate to use a holy language while enveloped in Tumah, which is our current status. The Rambam writes that a love song in Hebrew is more repulsive to Hashem than the same song in Arabic, for instance, because the pollution of the Holy language is an additional crime. If someone wants to store pornography in his house, thats bad enough. But to store it in the Aron HaKodesh is unspeakably worse. So to cause Loshon HaKodesh to be used as a street language, complete with all the disgusting ways it is used today in Israel, is just more of a reason why we should make sure it never gets into the streets. For our Creator to look down at the world and see His holy language – or even elements of it – used in magazines such as are sold in Kiosks on Yaffo or Dizengoff Street, or spoken by the lowest of the low trying to make a sale, is not something that he or we are happy about.
The Kuzari writes that Avrohom Avinu, therefore, spoke 2 different languages. One for holy speech – that was Loshon HaKodesh, and the other for mundane speech – that, the Kuzari says was some non-Jewish language that Avrohom Avinu took and changed around a little on his own. And thats the idea behind Yiddish. It is a non-Jewish language that we took and twisted a bit in order to make it exclusive among us.