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They are fonts that include information how to render those characters, but they also include rendering instructions for normal latin characters and other commonly used characters, because it is quite common to need both. It isn't like wingdings, where latin letters produce icons. Letter A is still rendered as A. You need to write actual Sanskrit characters to add them to your document. So you need to change your keyboard layout to something that produces those characters, copy them from somewhere or use alt-codes.

Computers handle text as list of numbers. On character is produced by sequence of one or multiple numbers in the list. Fonts define how these different numbers should be shown to the user, what kind of lines should appear on screen. So eg. 65 -> A. Unicode is a standard that defines which number means which character. Fonts include instructions to draw only some of the characters, because Unicode includes huge number of really weird characters and emojis. Sanskrit letters have different number codes than latin letters. Keyboard layout defines which number a key press produces.

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Why are they included as defaults? Shouldn't there be regional defaults? At this point, the majority of fonts in my drop-down list are non-English fonts, which makes it difficult to search for the fonts I want.

I don't know. I don't have the same issue.

It is possible that some software that you have used, installed tons of additional fonts for some reason.

You didn't specify OS or what languages you primarily type in, what language support you've installed, etc. But if your question is based on wanting to have only relevant fonts shown to you, there are ways in the OS to filter those or uninstall fonts you don't need.

This isn't really normal. I've got 2 Windows machines and 3 Macs and Office only has boring fonts in my language on them.

hmm, good to know. i've had two work macbooks over the past 15 yrs, from different companies, and both of them have all these other fonts.