“在监狱服刑”可以翻译为“serve a prison sentence” 或者 “be in prison serving a sentence”。
例如：
He is serving a prison sentence for robbery.（他因抢劫罪正在监狱服刑。）
She has been in prison serving a sentence for the past five years.（过去五年里，她一直在监狱服刑。）