There are two ways to work with multiple languages in Store Manager: if you have English version of the software, but running a store in some other language, or if you're using a non-English version of Store manager.
To change the language of your store simply press [CTRL+L] and select a language. All the data available in languages, other than the basic language of your store, will be displayed in separate tabs, columns etc. You can see Edit product page shown in an example here.
If you wish to switch the language of your Store Manager - go to Tools and select Application language you'd like to use.