When you install SU you get the option to choose additional language packs.
After the installation you cannot choose one of those languages manually.
One reason could be:
The mother language of a user is not english and he likes to take part in this forum. Than it would be helpful to switch to the english interface, because than he could use the correct names of functions and commands.
The user is working at a model together with a partner, who speaks another language than himself. It would be very helpful to be able to switch the language.
Please think about adding an option to change the language manually. Thanks.