。
She has a great interest in English.
Literal
She [topic-は] English [in-に] greatly interest [subject-が] exists.
~に興味がある ('to have interest in') is the textbook way to express a subject-matter interest. Here に marks the target of the interest, and 興味がある is the existence pattern — Japanese says 'interest exists [in/regarding X]' rather than English's 'have interest.' 大いに ('greatly') is an adverbial form of 大い ('great'); it's bookish and emphatic, contrasting with the more casual とても or すごく. The whole phrase ~に大いに興味がある leans formal — a CV line or speech pattern, not casual conversation.