。
She's good at English conversation.
Literal
She [topic-は] English-conversation [subject-が] skilled is.
英会話 ('English conversation') is a kanji compound treating 'conversation in English' as a single concept — common in the names of language schools and study programs (英会話スクール, 英会話教室). It contrasts with 英語 (the language as a body of knowledge): 英語ができる focuses on linguistic competence, 英会話ができる on the interactional skill of holding a conversation. The casual だ instead of polite です signals informal speech — a remark made between friends or family.