Think of language profiles as a "settings" profile. Since each language/region has different unwritten rules that define how a human "thinks" of words, shortening words, or even making intentional misspells, we can use language profiles to keep our isolated settings for higher accuracy and adaptability.
The Global Default profile is simply a generic profile that can be used as a "language-agnostic" profile.
In the Settings Tab, simply select the Language Profile for which you want to define your rules, then in the Wordlist Generation tab select any of the profiles you have set to use the rules of that profile.
Note: In this version of the tool, if you have no settings under a language profile and you use it in the wordlist generation, you most likely will get undesired outputs.