You need a UserBase account before you can be added to the Translator group, register or log in here adding your details to the bottom of the list. If you haven't already done so, consider adding your email address on your preferences page.
The workflow for translators is described in Translate a Page.
Be aware that your contribution will be governed by the Gnu Free Document License and Creative Commons License, see KDE UserBase:Copyrights for details.
I have translation right before. But right now when I save translations, it says unknown error happens. Do I need to apply translation right again?