Error message phrasing should not be in code. It should be in localization files, even for the default language, and not require a developer to modify.
Depends on the requirements. I've had many internal-only applications only require one language: English. At that point you can hard code many things, which skips the loading other files, checking different languages, etc.
It really depends on who‘s the target group. I hate localization in Developer Tools etc, because googling it is harder and a lot of times English is just easier to understand. It’s different for a shopping site.