Yeah, you're using `C` (hardcoded/builtin) locale, I can reproduce it with that. Try to open a command prompt, change directory to `C:\Program Files (x86)\Geany\bin` and run `set LANG=en` (or whatever language you use/want) and press enter, and then type `geany` and press enter.
Is it possible when you installed Geany that you didn't install the localization files? I believe they are selected for installation by default.