Following a discussion on the English community forums on this and a number of reports from users having issues with this, I'm posting this bug report:

When using the Office 365 plugins for SSO, the user's language preferences are being reset to 'en' for every time the user logs in. This can be found in the following code and should be resolved in some way:

in local/o365/classes/observers.php on line 271 we found the following code:
$updateduser->lang = 'en';

If a user's preferred language is anything but 'en' (like nl, es or fr), on logging in, the language preferences should not change to English.

Direct link to the discussion: https://moodle.org/mod/forum/discuss.php?d=358061