Here's why:
* The Summoner's Character: He's portrayed as a greedy, unscrupulous man who uses his position to extort money from people. He enjoys using his knowledge of Latin and legal jargon to intimidate others.
* Chaucer's Humor: Chaucer often used humor to expose the flaws and hypocrisies of his characters. It's likely that the Summoner's drunken ramblings would be a source of amusement for the audience.
* Medieval Drinking Culture: Drinking was common in medieval times, and people often became inebriated. This would have been a common topic for humor and satire.
While we don't have a specific record of what language the Summoner spoke when drunk, it's likely he would have become more rambunctious and less coherent in his usual mixture of Latin, English, and vulgar slang.