。
When it comes to French, she knows everything from A to Z.
Literal
She [topic-は] French [as-for-のことなら] what-from-what-until knows.
のことなら is the topic-establishing conditional: 'if it's about X / when it comes to X.' Built from のこと ('the matter of') + なら (conditional). It establishes a domain about which the predicate then makes a universal claim. 何から何まで ('from what to what,' i.e., 'everything from A to Z') is a fixed expression — a stock superlative for comprehensive knowledge or coverage. Together: 'when it comes to French, she knows everything you can name.'