If the site project needs to support multiple languages then Locale settings section can be used to add support for more than 140 languages.
By default a project default language is English (en). A project must have a default language, if the project supports a single language then that language is the default one.
Each project supported language adds a section under the Messages section, allowing to define language specific strings.
Global parameters are parameters that are similar with site urls parameters or site page parameters but are globals, meaning that can be accessed from any template script or controllers.
Values that are used in multiple places are good candidates to be stored in Global parameters. For example a web site will have the login, logout url links displayed on multiple pages, storing these url values in global parameters allows to change them in a single place.