Alexa is the free voiced AI assistant service that amazon provides from its cloud servers to devices that work with Alexa.
Skills that are available on Alexa enabled devices are often locational. For example, "Bus Timetable" skills for specific cities. Such skills are only relevant to a specific region. There are many such skills.
Many skills are also specific to languages that Alexa supports and only relevant and available where the service uses another language.
Skills such as Music streaming skills will be limited by regional and DRM licensing. There are music streaming or similar to BBC.
Some skills are global. For example, the native Radio streaming skill TuneIn will do it's best to provide radio streams from stations anywhere in the world when the local station broadcaster makes an internet stream available, (as all do).
Skills are created by interested skills makers in the regions where Echos and Alexa language-specific recognition is available.
If there are any skills useful in Greece depends on the skills makers in that region? I would think very few as the Greek-speaking population is small and that Alexa does not provide any Greek language recognition/interpretation.
I hope this helps. Apologies if this is a bit muddled.
I recently took an Amazon Echo out to my brother who lives in Turkey. He could not set it up as the app is not available in Turkey. However, I downloaded the app while I was there, set it up and it works fine. Not knowing much about them, we did think once I was back home, it would not work. Spoke to him yesterday and still working fine.