@Kissavo If you don’t remember examples of some of the non-greek words and phrases that was spoken in your region, then perhaps you could ask someone from the older generations in your community?

A lot of the ethnographic maps from the late 1800s/early 1900s do not depict any slavic populations in your region of Thessaly.  However, if there was a slavic population in that region of Thessaly, it may have Hellenized long ago.  For example, in Northern Greek Macedonia, slavic populations did exist and it has been documented through the ages, and the language is sparsely spoken today, but most living there identify as Greeks now.  I know for example, that the Greek Orthodox Church in Dad’s village in Northern Greek Macedonia was built as early as the 16th Century.  So a process of Hellenization to slavic populations in Modern day Greece could have occurred a long time ago.

In reference to your comment – “Moreover, the people coming from my place generally have blonde or light
brown hair and blue, green or light brown eyes, something not so usual
in Greece.” –

a) I know quite a few Greeks with blue eyes and they come from various parts of Greece, so I don’t think that is necessarily a slavic trait.  

b) I also know a few Greeks with Blonde hair from your region of Thessaly, but nonetheless, fair hair colour can be traced to depictions from Ancient Greece/Ancient Macedon in a number of mosaics found in Greece; so its not necessarily a slavic trait to have light hair, but it is a possibility to be a slavic trait also.

Mosaic found at Pella – Red/Blonde-hair: