Please read the man page on pacman-mirrors or the wiki which explains how it works.
Country is a category containing a number of mirrors - and the countries are read from mirrors.json file.
Continent is a category containing a number of countries.
Global is neither a country nor a continent but any and all mirrors at any given point in time and as such it does not make sense to implement your request.
You have the option of configuring your own mirror-pool - and such mirror pool is only ever changed by yourself.
So running e.g. fasttrack on a custom mirror pool will only include mirrors defined in that pool.
You can create such pool using the suggestion from @papajoke using the --country/-c argument followed by valid entries from the default pool defined by Manjaro.