Office 365 and SharePoint Online are multilingual. When a SharePoint Online site is configured for different languages, the user could select his display language in the profile settings. No configuration needed in the browser settings.
When you are in any SharePoint Online site, click the user symbol on the top left and click the “About me” link.
This will open the Delve page for the user. On the Delve page click the “Update Profile” button.
On the update profile page in the bottom you can see the link “How can I change language and regional settings?”. Clicking this link will display a short hint. In this hint click the “here” link.
A new tab will open in the browser displaying the details page from the user profile. Click the “…” on top and choose “Language and Region”.
On top of the page you find the section for the language preferences. Using the “Pick a new language” combobox you can select the preferred language. You can also remove the actually selected language.
When finished, click the “Save all and close” button at the bottom of the page.
That’s all. It might take some time, until the new setting is active.