If you configure a page layout which has a left column (e.g., "two column left" or "three column") then anything that you add into the left column will appear below the menu.
For example, that's how I added the google translate widget on the demo site.
To do this, go to your homepage and click Edit This Page, then click on the Layout tab. If necessary then click Create a new revision to make changes. Then under Current Template you can change which layout is used.
Once you have enabled a layout with a left column you can use the Custom includelet to place anything you like in there. If you set the includelet to inherit then it'll be visible on sub-pages too.
For more information see the Layouts section in the documentation.
Unfortunately the includelets won't be visible on non-CMS pages (i.e., inside apps) but it wil be there on the homepage and all content pages which is a good start!