I googled which Netflix series are available in Greek. For example, I was shown the series Alexa and Katie that allegedly existed in Greek. However, when I want to watch the series, there's not only no Greek dub but also no Greek subs.
Not all languages are directly available. Most of the time, it is limited to a few languages depending on the region.
For Greek you have to change the language to Greek. I think that can only be done via the browser.
Then you should also have the language in the app.