。
Forget French — she can't even speak her native language properly.
Literal
She [topic-は] French [topic-は] needless-to-say native-language [even-すら] properly cannot-speak.
This is a layered, formal-flavored sentence that piles up three concessive/scalar elements: 言うに及ばず ('needless to say, let alone'), すら ('even,' more emphatic than も), and ろくに+negative ('not properly, not adequately'). The combined effect is hyperbolic and slightly written: 'forget about French — even her own native language is below par.' すら is a literary/emphatic alternative to も in 'even X' constructions; 言うに及ばず is a fixed classical phrase still used in formal writing.