It works well and I get lots of fasts mirrors localized on my continent (Europe) without the global ones.
When I ask for new mirrors without localization option Global is in the list and faster than everyone else even those localized on my continent.
So, could you please add the global ones when we set continent ?
Please read the man page on pacman-mirrors or the wiki which explains how it works.
man pacman-mirrors
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.