1. It is the main aim to use a language that is to communicate with each other.
2. It is to communicate with each other that is the main aim to use a language.
Are these sentences grammatical?

No.

1. It is the main aim to use a language that is to communicate with each other.
The main aim of language is to communicate with each other.

Communication, with the exchange of information, has survival value for humans. It is the main aim of language.