Of course, this is always the case. Every user has her/his own environment, including her/his languages… In KDE you go to Configure Desktop > Regio > Languages (that is in KDE 3, but KDE 4 has similar) and there you choose your language.
The language should have been installed of course and that is something where root is responsible. YaST > Softwrae > Software management; in upper left change the Search button to Languages, choose your language(s) below it and install. After that the user has that language available for choice as per above.
You probably changed it in YaST then, and that is then the system wide default. But the user can always make his own choice not to use the deafult (if the others language is installed of course).