There is a possibility of using a "Field calculator" inside the Graphical Modeler.
Let's assume there is a point layer 'test' with some features in it, see the image below
Thereafter I created a simple model for reverse geocoding as following:
where a "Field calculator" possess the following parameters
In the 'Formula' tab apply this ...
IMHO the issue with non-ASCII characters shall not arise anymore, due to improvements that were done in software since 2017 year. Perhaps the problem was about the PyQGIS's version or OS settings.
Nowadays the geoding of 'Hacettepe Üniversitesi' object works well with both GeoPy (as was mentioned by @ivan) and "GeoCoding" Plugin using PyQGIS.
Since QGIS 3.18, no need for third party to geocode from Nominatim or Google Maps (not demonstrated below as you need an API key) as there is a class dedicated for this purpose in PyQGIS
Below, a recipe for Nominatim
from qgis.core import (
GeoServer has no geocoding functionality built in. There is however a number of open source geocoders out there, this site offers a list (not sure if it's complete or up to date, just sharing as an example):
If you need the GeoCoding to happen through a OGC compliant service, wrapping in a WPS process would be ...