How to change your display language in SharePoint Online / Office 365 / OneDrive

There are three settings which affect the display language in SharePoint Online.  Here is the order of preference from highest to lowest:

1. The user language preferences in a SharePoint Online user profile
2. The browser language preferences in FireFox or Internet Explorer
3. The language settings for a particular site.

 In order for a language to be selected, it must be activated in:

• Browser language preferences
• Site language settings

Setting the user language preferences in SharePoint Online is optional.  In addition, SharePoint Online can take several minutes to update the changes to the User Profile language preferences.

Examples:

1. User profile preferences: DE
2. Browser preferences: ES,EN,DE
3. Site language settings: ES, DE

The language displayed would be DE (German) because it is the first and only language available in the User Profile.

Another example:

1. User profile preferences: ES,EN
2. Browser preferences: EN,ES
3. Site language settings: EN

The language displayed would be EN (English) because it is the only language available in the site.

Change all user's default timezone setting for their MySite in SharePoint Online

In order to change the default timezone setting for the MySite in SharePoint Online:

1. Connect to SharePoint Online with an administrative account.
2. Go to the administrator's MySite.
3. Select Site Configuration from the menu.
4. Select Regional Settings from the Site Configuration screen.
5.  Choose the desired default timezone.  Click OK.
6. Done! All other users will inherit this default timezone.


To confirm the change:
1. Connect to SharePoint Online with a regular user account.
2. From the user name dropdown menu, select "About me"
3. From the user profile screen, click on "Edit profile".
4. Click on the three dot menu and select "Language and region"
5. In the section "Region", change the radio button "Choose your settings" from "Always use the regional settings defined by the site administrators" to "Always use my personal settings"
6.  Confirm that the default timezone in is the one chosen by the site administrator.
7.  Cancel out from the screen to leave the default setting.

Source: http://community.office365.com/en-us/f/148/t/175534.aspx