Grace cannot read Sydelle’s shorthand notes because they are Polish.

Grace Wexler can understand shorthand.  She tries to read Sydelle’s notes on the Westing will, and declares that they are gibberish.  Sydelle explains why Grace cannot read them.

Mr. Hoo explains that he did not steal her notes; he found them in the restaurant.

[Those] notes you selfishly dangled under our noses are completely worthless.  My partner knows shorthand and she says your shorthand is nothing but senseless scrawls.  Gibberish. (ch 11, p. 66)

Sydelle is angry, and she tells them that she wrote the shorthand in Polish so that no one can read it.  She explains that she could not trust them.

At this point, all of the heirs are suspicious of one another.  As this incident demonstrates, instead of working together they try to cheat and steal to get an advantage.