My German is very rudimentary (A1) and I have no regular exposure to the language except what I get listening to Bach. Recently, I have been very interested in Bach's cantatas (I have a huge box set by Gardiner) and I am in the habit of looking at the text with translation as I listen. I'm curious about the German that Bach used and how it compares to modern everyday German. Is it good learning material for me?
Views from a native German speaker, just having looked at a text booklet: the vocabulary used in the cantatas is slighty dusted, but still easily recognisable. A few words are dated and some have strange umlauts, where the modern counterparts have none. I guess in the church context one would notice fewer substantial changes. Without knowing which solists Gardiner had available, I would warn against taking the pronounciation too seriously, however - even well-known singers struggled astonishingly.
There are fairly good translation websites out there, for example http://www.bach-cantatas.com/Texts/IndexTexts-Eng3-BWV.htm
I'm native in both languages and it's actually a bit easier for me to read the English translation than the original German. It's dated and very "church-centric", i.e. uses phrasings and terms that are solely used in religious context. As such it would NOT make good learning material.
Poetry written for use in church hymns has a rather restricted subject matter and vocabulary to begin with. And in any language, the difference between rhyming poetry and regular speech is always quite different.
German has changed quite a bit since Bach's time. In particular, German vocabulary, though not the grammar, has changed dramatically just in the last 75 years, since the end of World War II. Since that time, German-speaking people have adopted a great deal of English words and "Germanicized" them ("eingedeutscht") to replace traditional German words. This is no doubt because of all the trade and cultural influence from the United States and England as Europe was re-built after the war.
I think there's certainly no harm in studying Bach's texts to help you learn "classical" German, but realize that the vocabulary of Bach's day is not the same as the vocabulary used today.
Presumably, the differences between modern German and the language used in Bach's Cantatas, when considering its usefulness as a tool for learning the language, is less significant than the rather specific nature of their subject matter. As much of the cantatas are based upon liturgical texts, using them as an aid to learning German, would be like learning to speak and read English by using the King James Bible! For this reason, I would imagine that useful, conversational German might not be improved as readily by studying these cantatas, as by using more conventional methods. But, as a means to understanding the differences between modern German and 18th Century German (and earlier German…), you could have a really interesting area of study. However, it is worth noting that the German used in the cantatas is itself from a number of different eras. Text from Bach's 18th Century contemporaries is set alongside the text of the Luther Bible (translated from Hebrew into German in the early 16th Century) and chorale texts.
As soon as I read your question, it reminded me of the central character of Aldous Huxley's Brave New World, who learns English by reading Shakespeare!
Bach's cantatas are essentially standard German. Where he uses direct Luther text citations as lyrics, you may have some Early New German, but far and wide it is pretty much indistinguishable from slightly stilted Modern German. Definitely quite more similar than even Shakespeare's Early Modern English is compared to Modern English.
So you should be fine with modern dictionaries and learning material. Pronunciation is also modern, and one doesn't try to put a Saxonian accent to it either: that would be considered a strange regionalism.