I use to set my Google spreadsheet's primary language to Chinese, then I changed it to English. Everything is changed to English except the error messages when I run my script. I have tried to set to other languages like French but the error messages are still in Chinese.
As have been mentioned by Robin, you can try to use local settings in spreadsheet:
File->Spreadsheet Settings
Or you can try to do it programatically and use setSpreadsheetLocale. That's all the language-related solution we can think of.