How can I present an argument about the following two different perspectives? 1. Dialects are related to a language 2. Dialect refers to a variety of language.

It's possible that the question is asking you to support one statement as true and the other statement as false. It's also possible that the argument could explain how each statement is true under certain conditions, and the argument then needs to explain in what situations each perspective might be true.
In my own opinion, I would argue that both statements are always true. Merriam-Webster defines dialect as a "regional variety of language." The definition specifically echoes the second perspective in that dialect offers variety. The variety is language based; therefore, dialect relates to language which is what the first perspective statement says. Maybe your argument needs to be a concise thesis statement that ties those two perspectives together. In that case, the following thesis statement might work.

The two dialect perspectives are so woven together that it would be erroneous to discuss one without the other.