“鼓舞士气的讲话”可以用英语表达为 "a motivational speech to boost morale" 或 "an inspiring talk to lift spirits"。以下是具体解析：
1、 "motivational speech to boost morale"
motivational（形容词）：激励的、鼓舞人心的
boost morale（动词短语）：提升士气、鼓舞斗志
示例：The general gave a motivational speech to boost morale before the battle.（将军在战斗前发表了鼓舞士气的讲话。）
2、 "inspiring talk to lift spirits"
inspiring（形容词）：鼓舞人心的、启发性的
lift spirits（动词短语）：振奋精神、提升情绪
示例：The coach delivered an inspiring talk to lift spirits before the game.（教练在比赛前发表了振奋人心的讲话。）
其他常见表达：pep talk（非正式）：鼓舞士气的简短讲话
示例：The manager gave the team a pep talk at half-time.（经理在中场休息时给球队做了鼓舞士气的讲话。）
rallying cry（比喻）：激励团队的号召或口号
示例：His speech became a rallying cry for the entire organization.（他的讲话成为了整个组织的激励口号。）
根据语境选择合适的表达即可。