Sb. believes that humans may be able to think about objects that have language associated with them

一种理解

1
。。。objects that have language associated with them

‘that have language associated with them’ 修饰 object

2

have 其实就是一般的动词。
A have B

而B正好是 language associated with them

‘ssociated with them’修饰 language
即为“与其有关系的语言” 在语法中整个可以当作一个名词

所以：

objects that have language associated with them 意思是： 有 与其有关系的语言的 物体（有与其有关系的语言的物体，先不管汉语上通不通顺）

that引导的定语从句
have实意动词，指objects have language
Sb. believes （that humans may be able to think about objects （that have language） associated with them）

第二个that是从句的主语，也就是修饰objects的从句“that have language associated with them”的主语，我引号中的那句就是一个完整的句子，用来修饰objects的

that引导的定语从句 have是实意动词