“second language” English has problems with articles, unusual plurals, irregular verbs, and tends to an overly formal tone

“only” English has problems with homophones, apostrophe placement, and using slang where it’s not appropriate

People who know other languages tend to model their sentence structure on their native language, and swap word for word. This can lead to conjugation issues, e.g. “I am feeling” instead of “I feel.”

People who only know English often base their understanding on the way language sounds, not how it’s written.