why would the debugger plugin use python code.
The debugger plugin is a UI to run the GDB debugger, which has Python embedded in it as an [extension language](https://sourceware.org/gdb/onlinedocs/gdb/Python.html#Python).
Clearly Geany and therefore the spawned GDB seem to be being run in an ASCII locale (probably the default "C" locale).
Are you starting Geany from the command line or a desktop menu? If menu try from the command line. Many menus do not run a shell, they just fork the program, so if you set the locale in one of your shell startup scripts it will not be set when Geany runs.