Here's an interesting letter on the subject of Yiddish, which was printed in Kerem Chemed 1 (1833). The actual action had taken place around 1815. The author was Yaakov Shmuel Bick, who was probably the only Galician maskil who was sympathetic to Chasidim. (I want to know which Chasid was sympathetic to the maskilim.)
What happened was, Mendel Lefin (probably most well known nowadays for adapting an excerpt from Benjamin Franklin's autobiography in a manner suitable for mussar) had made a Yiddish translation of Mishlei (Proverbs) for the benefit of the masses. This was naturally seen as uncouth pandering to the uneducated people by those who were at the time devoted to developing the prestige and capabilities of Hebrew on the one hand, and trying to promote and teach German, rather than Yiddish, on the other. One of them, a scholar named Toviah Feder (1760-1817) attacked Lefin in a pamphlet in which he accused him of terrible things, including pandering to women and whores and that Lefin had gone crazy in his old age. Yiddish? Feh. In the pamphlet he assembled a group of heavenly luminaries, including Rabbis Menasseh ben Israel, Moshe Chaim Luzzatto, and Moses Mendelssohn, who can't understand Lefin's debased Yiddish translation. Ben Israel and Luzzatto are really left scratching their heads, until finally an ignorant melamed is able to explain it. In addition, several other leading maskilim of the previous generation, such as Itzik Euchel, who translated Mishlei into German, men who personally knew and admired Lefin, are present in this fantasy scenario and feel puzzled and betrayed by Lefin's betrayal of enlightened ideals. The overarching attiude which Feder was expressing was that Yiddish is not a real language. What Lefin should have done was write in correct High German, especially considering that Lefin was highly educated, knowing German and French perfectly well. Was not his role to educate, not descend to the level of the lowly?
Lefin's friend Bik took up the cudgel to defend Lefin, and in doing so defended Yiddish as a legitimate language, with a lineage, usefulness, and a possible future. Here is his letter, followed by Feder's reply (scroll down to the end for my summary):
After praising and rebuking Feder, he quotes Jacques Basnage's History of the Jews that the foremost quality which God perceived in Moses was his empathy for the people!
He goes on to point out that the vernacular of the people is the proper medium for instructing them, noting that Tissot's famed medical book was translation in the various vernacular language of Europe; in fact, Lefin himself translated Tissot's book into Hebrew (for the benefit of Jews who knew no European language), and no less than the foremost of all translators, Mendelssohn himself, praised Lefin's translations.
For example, Cheshbon Hanefesh, which is based on a wonderful work by the scholar Benjamin Franklin of Philadelphia, in North America. This scholar is famed all over the world, and is a "Non-Jew who lit a lamp which is used as light for Israel." So R. Mendel undertook to present the good in this work to our people, and the rabbis in their approbations acknowledged that it was a lofty, new thing. And the book was well received by the people.
Now, he continues, the language of the Proverbs translation which, to you, is like chirping birds and braying cattle; do not forget that this is the language of our fathers and ancestors here in Poland for some four centuries! [Yiddish] is the tongue in which the ge'onim the Bach, Rema, Sema, and Shach spoke, thought and preached. This is the language with which we heard the Gaon of Vilna. Not only that, the scholar Fabre (?)* in his geographical work numbered this language as a daughter tongue of German. If German is the beloved firstborn to you, why don't you denigrate the Tzeena Reena and various other translations like it? These - which were very useful in their time - were written in a very debased German, with a very sparse vocabulary. None of those were able of making an impression on a tasteful person, like the translation of Lefin, who with wisdom clarified in the best methods of translators, even without the language having been improved as of yet.
He goes on to point out that there are vernacular newspapers throughout the Austrian-Hungarian empire, and this in the capital cities, where pure and proper German is used by the authorities! If this is so then certainly Jews who live isolated in Ukraine, who do not know how to read books in any other language. The way of scholars is out of good will and righteousness they do not neglect the people who need help, and they write works in their vernacular. This is in exchange for the hard work which the common people do to sustain their needs.
Note that French and English are also admixtures of Germanic, Gallic, Latin and Greek languages, and it was purified as a result of the work of scholars over these past 300 years. Now the most sublime poetry and the best thoughts can be expressed in these tongues, still mixed from many tongues. Only 100 years ago German was still debased. Eighty years ago Russian was a crude tongue. Even the classical languages, Greek and Latin, originally they were crude, until their scholars polished it, discoverd and expounded their grammatical principles, over generations, until they were perfected, and are the wonders we behold. The masses created the language! - and so it is in every nation. There's no difference in this stage between any language, all are originally somewhat crude, and then the sages take the formless mass (homer and golam) and shapie it, purify and perfect it.
He closes by telling him that if when all is said and done he still didn't like the translation, why heap abuse upon Lefin, rather than focusing on the defects in it? Do you have harsh words for the incomrehensible aggadot of Rabbah bar bar Hannah? It wasn't right to write such a pamphlet, and he ought to reach out to Lefin and ask for pardon.
Feder's reply, which you can see below as well, tries to be conciliatory, denying that such an illustrious maskil like Lefin could possibly have been wounded by his words. But he was says he will not to publish his pamphlet, which had already been paid for - if Bick pays or raises funds to reimburse him. And so it happened.
When all is said and done, eventually Bick's sympathies for the masses (which in his native Brody meant the Chassidim) led to a dramatic act of identification with them, not the misnagdim and the maskilim: he sold his copy of the Guide of the Perplexed! Allegedly, anyway. Here is the reference to this dramatic action, in a letter to Bik from Samson Bloch. I have my suspicion that Bloch meant it metaphorically although all the historians took it literally:
* I'm not exactly sure which Fabre (Fabro, Fabreaux, etc.) is intended or which work. He says he used the edition printed in Halle 1815.