It is two lines in Python:

from collections import Counter

print(sorted(((v, k) for k, v in Counter(open(fname).read().split()).items()), reverse=True)[:n])

(Yes, I tested it.)

Using Counter is standard for this sort of thing. It was introduced in Python 3.1 in 2009: doing without adds just one line (two more lines of code, one less line of import).