Office 365 is available in multiple language, so it means that SharePoint is also available in these available languages.

Ok, but when you create a SharePoint site collection, you choose ONLY one language for the interface. How can you allow your users to change (or let SharePoint automatically set) the language?

Logon to the SharePoint site using a site collection administrator account and go to the Site Settings

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Then click on Language settings link

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Enable the languages you want to provide to your users

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Starting then, your users are able to change the language to use; they just have to click on their name and choose the language to display. Note, SharePoint will also automatically used enabled language to display the SharePoint interface based on the browser settings (languages)

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By the way, this is the same for SharePoint 2010 On Premise