Hi cyril,
It's not the field name, it is the label which is important for the override.
See
here !
The field name is "username" the label is "Login Name".
SEBLOD will define the override constant: COM_CCK_LOGIN_NAME not COM_CCK_USERNAME.
In the Language Manager -> Overrides must now be defined this constant for each language that you need.
First Select the Filter <your language> -Administrator.
Language Constant: COM_CCK_LOGIN_NAME
Text: Your translation eg. Login Name
Marked the checkbox "For both locations" (the constante is then applied for backend and frontend)
Save it
If the language is not available for the constant, SEBLOD will then display the label.
So you can make the label in your language for other languages define the constante.
Paul