We don't really use "periphery" for the circumference in Greek, we use "perimeter". Periphery refers to something outside the object you're describing, whereas perimeter is right at the edge of the object, but usually belonging to it.

... and I've forgotten how to speak Greek :(

I've lived in the UK for the last 15 years. I even think in English, these days.

I've lived in Greece for the last 40 years and even I think in English (when I think in language).

For me it's to do with work. I think of work all the time and I find it difficult to think of it in Greek. So I end up thinking about work, in English, all the time.

The problem is that I keep forgetting Greek words and I find myself using English expressions even when I speak in Greek, even when I'm not speaking about work. And then I start saying things I'd never say in either language. I'm particularly shocked when I make blunders like "περιφέρεια του κύκλου". I just hope it's too early for early Alzheimer's.

Oh don't worry, this happens to me all the time too. It seems to be normal.