Ever since humans have lived on the earth, they have made use of various forms of communication. Generally, this expression of thoughts and feelings has been in the form of oral speech. When there is a language barrier, communication is accomplished through sign language in which motions stand  for letters, words, and ideas. 

【翻译】自从人类生活在地球上以来，他们就利用各种交际方式。一般来说，这种表达思想和情感的方式都是以口头讲话进行的。当存在语言障碍时，交际则要通过手语完成，手语中动作代表字母、词语和意思。

黑体部分结构分析：

When there is a language barrier（时间状语从句）, communication is accomplished （主句的主语和谓语）through sign language（方式状语） in which motions stand  for letters, words, and ideas（定语从句，修饰sign language）.