“郑重声言”常见的英文表达是 “solemnly declare” 。
“solemnly” 意思是“庄严地；严肃地” ，突出郑重、认真的态度；“declare” 有“宣布；声明；宣称”的意思，二者搭配能准确传达“郑重声言”的含义。
例句：The organization solemnly declares that it will take all necessary measures to protect the rights of its members.（该组织郑重声言，将采取一切必要措施保护其成员的权利。）