“多国参加的”可以翻译为“involving multiple countries” 、“participated by multiple countries” 或者 “with the participation of multiple countries” 。
例句：
This is an international conference involving multiple countries.（这是一场多国参加的国际会议。）
The project was participated by multiple countries.（这个项目有多国参加 。）
The military exercise was conducted with the participation of multiple countries.（这次军事演习有多国参加。）