Count strings in nested list

A simpler and more performant approach would be to flatten the lists using itertools.chain, and to count the strings with collections.Counter:

from collections import Counter
from itertools import chain

Counter(chain.from_iterable(sentences))

Counter({'my': 3,
         'first': 1,
         'question': 1,
         'in': 1,
         'stackoverflow': 1,
         'is': 2,
         'favorite': 2,
         'language': 1,
         'python': 1})