The Jungle is a passionate plea for the reader's compassion in light of the injustices that ruin the lives of Chicago's poor slum residents. So language is extremely important: Sinclair is using words to try and convince you of his point of view. At the same time, a huge bulk of the novel casts doubt on the ability of language to communicate anything important. From Teta Elzbieta's inability to read and understand the family's disastrous house deed to Jurgis's own lies about Republican candidate "Scotty" Doyle, The Jungle abounds with examples of characters either being deceived or deceiving others for personal profit. What is more, there are multiple languages being spoken in this novel, from Jurgis and his family's Lithuanian to the rough Chicago street slang that Vilimas and Nikolajus learn downtown. So, even when these characters aren't deliberately lying to one another, there is still plenty of room for misunderstanding and mistranslation between them. Language becomes yet another obstacle to worker organization and mutual cooperation.
Upton Sinclair only emphasizes Jurgis's accented English when he wants to draw attention to misunderstanding between Jurgis and people who will take advantage of his ignorance; otherwise, Jurgis's English dialogue reads like a native speaker of the language.
Upton Sinclair uses scare quotes to draw attention to Packingtown terms and nicknames like "slunk" calves and "Bush" Harper. By using quotation marks in these contexts, he is making Chicago's slang and its speakers seem as alien to the reader as the Lithuanian words Sinclair reproduces in other parts of the novel.