What does the last paragraph mean?
'Serge!' said Toby, 'shut up!'
But the wild babble poured out of Serge's ravaged face without a pause.
'Serge! Do you hear! Stop it!' Toby said, and pushed Ratoff lightly on the shoulder so as to urge him away from Searle.
This was the final touch, and Serge's voice rose into one high continuous stream of vituperation, most of it couched in mercifully unintelligible English but spattered liberally with phrases in French or Spanish and studded here and there with epithets and descriptions of a freshness that was delightful. 'You middle-west Lucifer!' was one of the better ones.
Q1: What is 'mercifully unintelligible English'?
Q2: what is 'a freshness that was delightful'?
Q3: What is 'middle-west Lucifer?

Question

Answer

1) From the previous part of the sentence it states that serge is using a lot of foul language (high continuous stream of vituperation), so the listener is grateful that most of it is unintelligible (hence mercifully unintelligible)
2)  He seems to be commenting on how some of the foul language/insults being used are very creative.
3)  Lucifer, is a reference to the devil.  I don't know what the middle-west refers to.