“当法官”常见的英文表达是 “serve as a judge” 或 “act as a judge”。
“serve as a judge” 强调担任法官这一角色，履行相应职责，较为正式、常用。例如：He has served as a judge in the local court for ten years.（他在当地法院担任法官已有十年了。）
“act as a judge” 同样表示充当法官角色开展工作， “act” 更侧重于行为、行动方面，不过在实际使用中，两者差异不大。例如：She was asked to act as a judge in the talent show.（她被邀请在才艺表演中担任评委/法官角色 。）