Looking at the quote in context, we find that Jane overheard Mary addressing her sister thus:
'Listen Diana, Franz and old Daniel are together in the night-time, and Franz is telling a dream from which he has awakened in terror - listen!'
She then reads the account, a portion of which is then repeated by Diana, although it is not understood by Jane at that time.
However, later in the book, after Jane receives her uncle's inheritance, she lives with her cousins, the Rivers, for a while. During this time, she works at learning German, until her cousin St.John Rivers persuades her to learn Hindustani instead.
No doubt Jane asks Mary and Diana which German book contains the characters of Franz and Daniel, and is able to identify the quote in question.
According to the story, the Rivers sisters are learning German by working their way through German literature 'with no master but a lexicon.'
The line in question makes a deep impression on both sisters. The only way I can see that Jane would know it was the right quote would be by relying on the memories of Diana and Mary as to what they were reading the night in question.
It is possible to remember one or two words spoken in a foreign language, or at least how they generally sounded, even if you can't remember the whole sentence. I can accept that Jane could have remembered the word Nacht at the end of the quote, for example.