Even though she knows more than one language, Sonia sometimes messed up when she's talking to her classmates. She can also tend to forget or not notice when she talks in another language that her classmates don't understand when she's excited, flustered, or angry, then if she sees their confused face she'll realize that she's speaking in another language, try to translate it back into Japanese, but she has already forgot what she was trying to say and she gets so embarrassed.
This has happened more than once when she talks to Gundam.
