there is one language used in every country in the world为什么不

是的,因为there be 句型中是属于倒装,真正的主语是one language
如果不用倒装,正常语序就是,
one language is used in every country
因此,按照主谓一致的原则,谓语动词看主语,主语是单数,谓语也用单数