It's possible to create constants or similar and use them inside a node description or title in Drupal 7 Let's say we have a download page in a multilingual Drupal site, and we want to define the download version, for example '1.1.0' and a download URL 'http://myweb.com/setup_1_1_0.exe', to use it on the distinct languages nodes for the download page, inserting them as a tag or similar on the tittle and body of the node.

How could it be done in Drupal 7?
I'm not sure I fully understand your need. But I would try to set a token called current_software_version as 1.1.0. Node titles and descriptions support the use of Tokens.

Tokens may also be supported within links, when using contributed modules or the redirect module I think.

You typically make custom tokens within a custom module you create. If you want an easier way to make the token value and name, I would look at the Variable module to build a UI and Admin Edit page for your current_software_version value. The Variable module will provide a token of the value for use throughout the site for you.