。
She's hooked on learning French.
Literal
She [topic-は] French's mastery [in-に] is-absorbed.
熱中する ('to be absorbed in, engrossed in, hooked on') is a Sino-Japanese compound — 熱 ('hot, fever') + 中 ('inside, middle'). The metaphor is compelling: the person is 'inside the heat' of an activity. The に marks the focus or target of absorption — the canonical particle for verbs of mental engagement (集中する, 没頭する, 熱中する all take に). 習得 ('mastery, acquisition') here functions as a noun, with の linking it to 'French.'