Complex sentence: Thalberg had in December 1838, during his stay in Leipzig, had already announced that he would take time off after the end of his tour, and did not perform at any concert during his stay in spring 1840 in Paris.
Split into simpler sentences: Thalberg had in December 1838, during his stay in Leipzig, already announced that he would take time off after the end of his tour. He did so, and did not perform at any concert during his stay in spring 1840 in Paris.