How should I complete the "Statement of the problem" part of my thesis proposal if it there are very few similar works in the literature? I don't know how should I state the lacuna in the discipline. If there were researches worked on my topic before, I could cite them and say that they have worked on A, B, C, and I want to work on D, but there is not.

My MA thesis is sort of a following to a PhD research conducted 4 years ago (but not following one of those 'suggested for further research' questions, of course), and the topic is relatively new.

To help me with writing this part, what questions can I ask myself and find answer to?

I major in Translation Studies, by the way.
For my money, you won't be able to describe your contribution in terms your audience will understand until you can contextualize it in related work. Literature review is both an opportunity to adopt the established language of other researchers, and one to sharpen the contours of your idea.

Cast your net a bit wider. Certainly related work exists, and once you find it, you will see lots of ways to compare and distinguish your ideas from those of the authors that came before you.