Interpersonal relationships can develop a unique language, thus creating a ‘culture’ of two

The best answer to this question: Interpersonal relationships can develop a unique language, thus creating a "culture" of two, would be: true.

Explanation:

Although the term "culture" relates to many more factors than just language, the fact that two people develop such a unique language, so unique that it only involves the two of them, means that they share a series of conditions that allow them to do so. In this respect, that language makes them both unique and differentiates them from other people, or groups. It shows that these individuals that share an interpersonal relationship also share common customs, traditions, beliefs, and thus from that they can also generate a common language with which only the two of them will understand each other. That´s why the answer is true.