Recently a customer asked me to look into the question of language choice and Office 365.  In short, it’s complicated.

• A user can go to My Profile (top right portion of screen) in Office 365 and change their language preference.  This changes the elements of the Office 365 Home page, but doesn’t change the elements of applications such as Outlook or SharePoint.  I tried this (using Spanish, since I have some knowledge there) and found, for instance, that the control for getting to SharePoint Online management was changed to Spanish, but the actual SharePoint Online management page was still in English.
• SharePoint Online has its own settings for language.  These can be changed, but essentially require a re-build of SharePoint Online content, done via service request to Microsoft.  So it’s not a trivial matter.
• Some elements of language appear to be controlled by the OS and are therefore computer-dependent.  This requires changing the settings within Windows, as well as (in at least some cases) installing language packs from Microsoft.
• Some elements just involve changing the keyboard used for Office, such as to a Japanese style keyboard.  This is mainly to allow for entry of characters particular to that language.

So there’s no command or setting that “automagically” changes the UI to another language.  It’s really a matter of understanding what elements you want to change, and where those elements are controlled.