They have all been languages that were used by people in man

先调整下顺序：
They all have been languages that were used by people in many different countries.
that引导一个定语从句修饰前面主句的languages.
后面 从句中 were used by 为被动语态 意为：被...使用；
people后面的in many different countries 是一个介词短语坐了后置定语’可以译为：许多不同国家的人们
综合下来大概意思是：他们（根据上文翻译）都已经成为了被不同国家人民使用的语言.