。
She's speaking French, so I have no idea what she's saying.
Literal
She [topic-は] French [in-で] speaks [because-ので] what [object-を] is-saying [explanatory-のか] does-not-understand.
で here marks the medium or means of speaking — 'in French.' ので gives the soft circumstantial reason ('so'). The embedded question 何を言っているのか is the standard frame for 'what (someone) is saying' as a noun-clause: question word + 言っている + のか. The のか combines the explanatory の with the question particle か, marking it as an embedded indirect question. わからない ('not understand') is the negative potential of 分かる, common in describing the speaker's failure to grasp.