Stacks are re-used blocks. They help speed up your website, and reduce duplication of content.
They can be found in Stacks area via your Dashboard.
Global Areas - these are elements on your website that exist on every type of that page template.
E.g. Most items at the top of your website (the header) are Global - they appear on every page of your website.
If you have a multi-lingual site, you might need to localise your stacks. For example, provide a German translation of your Footer stacks. This is quite simple to do in Concrete5 CMS. The CMS automatically knows what stack to load. You can provide the EN stack in default, you don't have to create an EN version.
E.g. Mobile Main Menu Navigation - this may have been created as a separate stack. That gives you the flexibility to have a reduced, more succinct, mobile navigation menu should you wish.
Login to your website
Go to Stacks
Go to Global Areas
Select Mobile Nav
Edit the block
Provide a localised version of the stack
IMPORTANT: make sure you click "Publish Changes" button to see the updates on your website.