I am Greek, and when I lived in Greece and had my system language to Greek, the first thing I would usually do would when installing a piece of software or visiting a website that realised the fact that I’m Greek would be to ensure it’s set to English. The reason is partly because that’s how most people would use the software in question so I would be able to Google how to do something more easily, but also because sometimes the Greek localisation is… more difficult to understand than if it were just in English. Some things I’ve seen throughout my life:

• Terrible, incomprehensible grammar thanks to using automated translation software to translate your UI (Don’t do this!)
• Direct translation of date formats, ending in something that would directly trnaslate as “Of November 11”, which is about as right in Greek as it is in English.
• Doing half a job, and making it obvious. Non-localised text in the middle of localised text, localised text that hasn’t been updated this century (ISTR the University of Piraeus’ English page mentioning really ancient hardware, while their Greek page didn’t. While I don’t doubt they do have really ancient hardware, I think their failure to remove the evidence from their English page is due to them never updating that page), or straight out leading a user to a non-localised screen without letting them know.
• Stuff like this:

What are your favourite localisation fails? Let me know!