How can I change the user interface from Greek to English and vice versa?

To change Cinobo's language on a browser, find the GR/EN indicator in the top right and select your preferred language or if you are logged in, click on the three lines symbol in the top right and select your preferred language there with the GR/EN indicator. On mobile, you'll find the GR/EN indicator inside the menu, top right.

On the app (smart TV, Apple TV, smartphone or tablet), the interface automatically follows your device's language settings, so you'd need to change your device's language to switch it. Changing the interface language doesn't affect your subtitle language options.