If it’s working for the smaller fonts but not the larger ones, then I think it may be because the language file only includes greek characters for the smaller fonts.
You would have to go back to the creator of the language file and ask them to add support for larger fonts. I found a few archival-type resources that have different Greek language packs that you can try to install:
Language Packs · MarSoft/pebble-firmware-utils Wiki
Pebble Language Packs - Gadgetbridge
There is also this recent project for generating language packs:
Pyxzure/pebble-lang-gen: Python utility to help create a custom language pack file for the Pebble watch (.pbl) (Language Pack Creator : r/pebble)