How do you narrow down a subject/topic for PhD study?

I'm looking into PhD courses and supervisors at the moment and I'm struggling to find areas that I both know enough about, and which aren't oversaturated already with research. I've studied French (BA) and Translation (MA), and while I have a few essays and research topics that I could probably build on, I still feel like I've spread my study a bit too thinly and don't have an area of expertise or deep knowledge. Has anyone else come from a master's feeling like they still haven't found a narrow enough area to go into PhD research, and how did you go about deciding on a research course/topic?