。
She'll probably be happy when she gets engaged.
Literal
She [topic-は] get-engaged [if/when-たら] happy will-become probably.
The conditional ~たら attaches to past-tense verb forms (婚約したら) to mean 'if/when X happens.' It's the most flexible Japanese conditional, covering both real and hypothetical futures. The polite conjectural でしょう closes the sentence with a softened prediction. 婚約 ('engagement to be married') uses 婚 ('marriage') + 約 ('promise') — literally 'marriage promise.' 幸福 is the more abstract, formal Sino-Japanese word for happiness, common in written register and weighty discussions of life.