Hi Kenny,
Did you try to set default editing language in other Office programs? e.g. Excel, Word
First of all, please refer to these steps and notes about setting default editing language in Office.
Set the default editing language in Office
1.Open a Microsoft Office program, such as Word.
2.Click File > Options > Language.
3.In the Set the Office Language Preferences dialog box, under Choose Editing Languages, select the editing language that you want to use, click the name of the language that you want to be the default language, click Set as Default.
4.After you change the default editing language, you need to close all your Office programs, and then open them again for the change to take effect.
Notes:
If for some reason, your default display or Help language is not available, Microsoft Office uses the next language in the list that is available.
If the language that you want is not listed, you might need to add more language services by clicking Add additional Editing Languages.
If your issue still exists, I suggest you run Project 2010 as administrator and then try to change the default editing language.
Please try these steps and check if they work for you.
Any updates please let me know, I'm glad to help and follow up your reply.
Regards,
Emi Zhang
TechNet Community Su