Stumbling block for nonnative speakers WSJ Crossword Clue

Think of the colorful expressions that don't appear in textbooks but are heard on the streets.

????? is informal language consisting of words and expressions that are not considered standard in the speaker'?????uage.

The clue refers to a common difficulty faced by people learning a new language. ????? consists of informal words and phrases that are often not taught in language classes, making it a 'stumbling block' for nonnative speakers.

"Ride" or "whip," for "car", Translating challenge, Lit for fun, e.g., Informal language, "That slaps," for something good, e.g.