Once you have opened the Management page, select 'Settings' along the top of the screen and select 'Languages...' from the menu.
Select the language you require from the list and press 'OK' to save. For the change to take effect, Management will need to be closed and reloaded.
Please be aware this will not change the language for the Homepage, this is only for the Management side of the system.
If the language you require is not on this list, you can request it by submitting a feature request at our ideas portal.
Please note that some field translations may be missing for some languages.